chore: add Dependabot security-only update config

.github/dependabot.yml

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+version: 2
+updates:
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 0
+    labels:
+      - "security"
+      - "dependencies"
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 10
+    labels:
+      - "security"
+      - "dependencies"

.github/workflows/dependabot-automerge.yml

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+# Dependabot auto-merge workflow
+#
+# Requires repository secrets:
+#   APP_ID         — GitHub App ID with contents:write and pull-requests:write
+#   APP_PRIVATE_KEY — GitHub App private key
+#
+# Auto-approves and enables auto-merge for Dependabot PRs that are:
+#   - GitHub Actions updates (patch or minor version bumps)
+#   - Security updates for any ecosystem (patch or minor)
+#   - Indirect (transitive) dependency updates
+# Major version updates are always left for human review.
+# Uses --auto so the merge waits for all required CI checks to pass.
+#
+# Safety model: application ecosystems use open-pull-requests-limit: 0 in
+# dependabot.yml, so the only app-ecosystem PRs Dependabot can create are
+# security updates. This workflow adds defense-in-depth by also checking
+# the package ecosystem.
+name: Dependabot auto-merge
+
+on:
+  pull_request_target:
+    branches:
+      - main
+
+permissions: {}
+
+jobs:
+  dependabot:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: read
+    if: github.event.pull_request.user.login == 'dependabot[bot]'
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@ffa630c65fa7e0ecfa0625b5ceda64399aea1b36 # v2
+        with:
+          github-token: "${{ secrets.GITHUB_TOKEN }}"
+
+      - name: Determine if auto-merge eligible
+        id: eligible
+        run: |
+          UPDATE_TYPE="${{ steps.metadata.outputs.update-type }}"
+          DEP_TYPE="${{ steps.metadata.outputs.dependency-type }}"
+          ECOSYSTEM="${{ steps.metadata.outputs.package-ecosystem }}"
+
+          # Must be patch, minor, or indirect
+          if [[ "$UPDATE_TYPE" != "version-update:semver-patch" && \
+                "$UPDATE_TYPE" != "version-update:semver-minor" && \
+                "$DEP_TYPE" != "indirect" ]]; then
+            echo "eligible=false" >> "$GITHUB_OUTPUT"
+            echo "Skipping: major update requires human review"
+            exit 0
+          fi
+
+          # GitHub Actions version updates are always eligible
+          # App ecosystem PRs can only exist as security updates (limit: 0)
+          echo "eligible=true" >> "$GITHUB_OUTPUT"
+          echo "Auto-merge eligible: ecosystem=$ECOSYSTEM update=$UPDATE_TYPE"
+
+      - name: Generate app token
+        if: steps.eligible.outputs.eligible == 'true'
+        id: app-token
+        uses: actions/create-github-app-token@f8d387b68d61c58ab83c6c016672934102569859 # v3
+        with:
+          app-id: ${{ secrets.APP_ID }}
+          private-key: ${{ secrets.APP_PRIVATE_KEY }}
+
+      - name: Approve and enable auto-merge
+        if: steps.eligible.outputs.eligible == 'true'
+        run: |
+          gh pr review --approve "$PR_URL"
+          gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ steps.app-token.outputs.token }}

.github/workflows/dependency-audit.yml

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+# Dependency vulnerability audit
+#
+# Auto-detects ecosystems present in the repository and runs the appropriate
+# audit tool. Fails the build if any dependency has a known security advisory.
+#
+# Add "dependency-audit" as a required status check in branch protection.
+#
+# Pinned tool versions (update deliberately):
+#   govulncheck v1.1.4 | cargo-audit 0.22.1 | pip-audit 2.9.0
+name: Dependency audit
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  detect:
+    name: Detect ecosystems
+    runs-on: ubuntu-latest
+    outputs:
+      npm: ${{ steps.check.outputs.npm }}
+      pnpm: ${{ steps.check.outputs.pnpm }}
+      gomod: ${{ steps.check.outputs.gomod }}
+      cargo: ${{ steps.check.outputs.cargo }}
+      pip: ${{ steps.check.outputs.pip }}
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+
+      - name: Detect package ecosystems
+        id: check
+        run: |
+          # npm — look for package-lock.json anywhere (excluding node_modules)
+          if find . -name 'package-lock.json' -not -path '*/node_modules/*' | grep -q .; then
+            echo "npm=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "npm=false" >> "$GITHUB_OUTPUT"
+          fi
+
+          # pnpm — look for pnpm-lock.yaml anywhere
+          if find . -name 'pnpm-lock.yaml' -not -path '*/node_modules/*' | grep -q .; then
+            echo "pnpm=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "pnpm=false" >> "$GITHUB_OUTPUT"
+          fi
+
+          # Go modules — detect via go.mod (not go.sum, which may not exist)
+          if find . -name 'go.mod' -not -path '*/vendor/*' | grep -q .; then
+            echo "gomod=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "gomod=false" >> "$GITHUB_OUTPUT"
+          fi
+
+          # Cargo — detect via Cargo.toml anywhere (lockfile may not exist for libraries)
+          if find . -name 'Cargo.toml' -not -path '*/target/*' | grep -q .; then
+            echo "cargo=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "cargo=false" >> "$GITHUB_OUTPUT"
+          fi
+
+          # Python — detect pyproject.toml or requirements.txt anywhere
+          if find . -name 'pyproject.toml' -not -path '*/.venv/*' -not -path '*/venv/*' | grep -q . || \
+             find . -name 'requirements.txt' -not -path '*/.venv/*' -not -path '*/venv/*' | grep -q .; then
+            echo "pip=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "pip=false" >> "$GITHUB_OUTPUT"
+          fi
+
+  audit-npm:
+    name: npm audit
+    needs: detect
+    if: needs.detect.outputs.npm == 'true'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+
+      - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4
+        with:
+          node-version: "lts/*"
+
+      - name: Audit npm dependencies
+        run: |
+          # Audit each package-lock.json found in the repo
+          status=0
+          while IFS= read -r dir; do
+            echo "::group::npm audit $dir"
+            if ! (cd "$dir" && npm audit --audit-level=low); then
+              status=1
+            fi
+            echo "::endgroup::"
+          done < <(find . -name 'package-lock.json' -not -path '*/node_modules/*' -exec dirname {} \;)
+          exit $status
+
+  audit-pnpm:
+    name: pnpm audit
+    needs: detect
+    if: needs.detect.outputs.pnpm == 'true'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+
+      - uses: pnpm/action-setup@a7487c7e89a18df4991f7f222e4898a00d66ddda # v4
+
+      - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4
+        with:
+          node-version: "lts/*"
+          cache: "pnpm"
+
+      - name: Audit pnpm dependencies
+        run: |
+          # Audit each pnpm-lock.yaml found in the repo
+          status=0
+          while IFS= read -r dir; do
+            echo "::group::pnpm audit $dir"
+            if ! (cd "$dir" && pnpm audit --audit-level low); then
+              status=1
+            fi
+            echo "::endgroup::"
+          done < <(find . -name 'pnpm-lock.yaml' -not -path '*/node_modules/*' -exec dirname {} \;)
+          exit $status
+
+  audit-go:
+    name: govulncheck
+    needs: detect
+    if: needs.detect.outputs.gomod == 'true'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+
+      - uses: actions/setup-go@d35c59abb061a4a6fb18e82ac0862c26744d6ab5 # v5
+        with:
+          go-version: "stable"
+
+      - name: Install govulncheck
+        run: go install golang.org/x/vuln/cmd/govulncheck@v1.1.4
+
+      - name: Audit Go dependencies
+        run: |
+          status=0
+          while IFS= read -r dir; do
+            echo "::group::govulncheck $dir"
+            if ! (cd "$dir" && govulncheck ./...); then
+              status=1
+            fi
+            echo "::endgroup::"
+          done < <(find . -name 'go.mod' -not -path '*/vendor/*' -exec dirname {} \;)
+          exit $status
+
+  audit-cargo:
+    name: cargo audit
+    needs: detect
+    if: needs.detect.outputs.cargo == 'true'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+
+      - uses: dtolnay/rust-toolchain@stable
+
+      - name: Install cargo-audit
+        run: cargo install cargo-audit@0.22.1 --locked
+
+      - name: Audit Cargo dependencies
+        run: |
+          # cargo audit operates on Cargo.lock at workspace root
+          # For workspaces, a single audit at root covers all crates
+          status=0
+          while IFS= read -r dir; do
+            echo "::group::cargo audit $dir"
+            if ! (cd "$dir" && cargo generate-lockfile 2>/dev/null; cargo audit); then
+              status=1
+            fi
+            echo "::endgroup::"
+          done < <(find . -name 'Cargo.toml' -not -path '*/target/*' -exec dirname {} \; | sort -u)
+          exit $status
+
+  audit-pip:
+    name: pip-audit
+    needs: detect
+    if: needs.detect.outputs.pip == 'true'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4
+
+      - uses: actions/setup-python@a26af69be951a213d495a4c3e4e4022e16d87065 # v5
+        with:
+          python-version: "3.x"
+
+      - name: Install pip-audit
+        run: pip install pip-audit==2.9.0
+
+      - name: Audit Python dependencies
+        run: |
+          status=0
+          # Audit each Python project found in the repo
+          while IFS= read -r dir; do
+            echo "::group::pip-audit $dir"
+            if [ -f "$dir/pyproject.toml" ]; then
+              if ! pip-audit "$dir"; then
+                status=1
+              fi
+            elif [ -f "$dir/requirements.txt" ]; then
+              if ! pip-audit -r "$dir/requirements.txt"; then
+                status=1
+              fi
+            fi
+            echo "::endgroup::"
+          done < <(
+            {
+              find . -name 'pyproject.toml' -not -path '*/.venv/*' -not -path '*/venv/*' -exec dirname {} \;
+              find . -name 'requirements.txt' -not -path '*/.venv/*' -not -path '*/venv/*' -exec dirname {} \;
+            } | sort -u
+          )
+          exit $status