ci: auto-tag releases when Cargo.toml version changes on main

Adds a workflow that reads the version from Cargo.toml on pushes to
main, and creates a git tag if one doesn't already exist. This triggers
the existing release workflow automatically, so merging a version bump
PR is all that's needed to cut a release.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sachiniyer

.github/workflows/auto-tag.yml

@@ -0,0 +1,37 @@
+name: Auto-tag release
+
+on:
+  push:
+    branches: [main]
+    paths: [Cargo.toml]
+
+permissions:
+  contents: write
+
+jobs:
+  tag:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Read version from Cargo.toml
+        id: version
+        run: |
+          version=$(cargo metadata --no-deps --format-version 1 | jq -r '.packages[0].version')
+          echo "version=$version" >> "$GITHUB_OUTPUT"
+          echo "tag=v$version" >> "$GITHUB_OUTPUT"
+
+      - name: Check if tag exists
+        id: check
+        run: |
+          if git ls-remote --tags origin "refs/tags/${{ steps.version.outputs.tag }}" | grep -q .; then
+            echo "exists=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "exists=false" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Create and push tag
+        if: steps.check.outputs.exists == 'false'
+        run: |
+          git tag "${{ steps.version.outputs.tag }}"
+          git push origin "${{ steps.version.outputs.tag }}"