ci: add workflow to close PRs inactive for 2+ weeks (#253)

## Summary
- Adds `.github/workflows/stale.yml` that uses `actions/stale@v9` to
close PRs inactive for 14 days (7-day stale warning + 7-day close
window).
- Mirrors the `usedetail/detail` stale workflow pattern, scaled from 30
→ 14 days.
- PRs labeled `do-not-close` are exempt. Runs daily at 00:00 UTC; also
dispatchable manually.

## Test plan
- [ ] Trigger the workflow manually via `workflow_dispatch` and confirm
it runs without errors
- [ ] Verify an open PR with no `do-not-close` label and no activity for
7+ days gets the `stale` label and warning comment
- [ ] Verify a PR labeled `do-not-close` is skipped
- [ ] Confirm PRs with activity inside the 14-day window are left alone

🤖 Generated with [Claude Code](https://claude.com/claude-code)
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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sachiniyer

.github/workflows/stale.yml

@@ -0,0 +1,26 @@
+name: Close stale PRs
+
+on:
+  schedule:
+    - cron: "0 0 * * *"
+  workflow_dispatch:
+
+permissions:
+  issues: write
+  pull-requests: write
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/stale@v9
+        with:
+          days-before-pr-stale: 7
+          days-before-pr-close: 7
+          stale-pr-message: "This PR has been inactive for 7 days and will be closed in 7 days if there is no activity."
+          close-pr-message: "Closing this PR due to inactivity. Feel free to reopen if still needed."
+          stale-pr-label: stale
+          exempt-pr-labels: do-not-close
+          operations-per-run: 1000
+          days-before-issue-stale: -1
+          days-before-issue-close: -1