ci: add workflow to auto-close stale PRs (#409)

## Summary
- Adds a daily workflow that auto-closes PRs inactive for 30+ days
- Skips PRs authored by jdx or labeled `keep-open`
- Includes different close messages depending on CI status (failing vs
passing)

Ported from jdx/mise.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: adds a standalone GitHub Actions workflow that only closes
eligible open PRs and does not affect application/runtime code.
> 
> **Overview**
> Adds a new GitHub Actions workflow (`.github/workflows/pr-closer.yml`)
that runs daily (and manually) to close PRs with no activity for 30+
days.
> 
> The job filters out PRs authored by `jdx` or labeled `keep-open`, and
posts a different close comment when CI checks are failing vs passing.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
959b5b27b2. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

.github/workflows/pr-closer.yml

@@ -0,0 +1,31 @@
+name: pr-closer
+
+on:
+  schedule:
+    - cron: "0 0 * * *" # daily at midnight
+  workflow_dispatch:
+
+jobs:
+  close-stale-prs:
+    runs-on: ubuntu-latest
+    permissions:
+      pull-requests: write
+    steps:
+      - name: Close stale PRs
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          gh pr list -R "${{ github.repository }}" --state open --json number,author,labels,updatedAt,statusCheckRollup --limit 100 | \
+          jq -r '.[] | select(
+            (.updatedAt | fromdateiso8601) < (now - 30*24*60*60) and
+            .author.login != "jdx" and
+            ([.labels[].name] | index("keep-open") | not)
+          ) | [.number, (if (.statusCheckRollup | length > 0) and (.statusCheckRollup | any(.conclusion == "FAILURE" or .conclusion == "failure")) then "failing" else "passing" end)] | @tsv' | \
+          while read -r pr status; do
+            echo "Closing PR #$pr (checks: $status)"
+            if [ "$status" = "failing" ]; then
+              gh pr close "$pr" -R "${{ github.repository }}" -c "This PR has been open for more than 30 days without activity. Note: CI checks were failing, which may be why it wasn't reviewed. Feel free to reopen or create a new PR if you'd like to continue working on this."
+            else
+              gh pr close "$pr" -R "${{ github.repository }}" -c "This PR has been open for more than 30 days without activity. Feel free to reopen or create a new PR if you'd like to continue working on this."
+            fi
+          done