ci: add workflow to auto-close stale PRs

Closes PRs that have been inactive for 30+ days, aren't authored by jdx,
and don't have a "keep-open" label. Runs daily at midnight.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/pr-closer.yml

@@ -0,0 +1,29 @@
+name: pr-closer
+
+on:
+  schedule:
+    - cron: "0 0 * * *" # daily at midnight
+  workflow_dispatch:
+
+jobs:
+  close-stale-prs:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Close stale PRs
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          gh pr list -R jdx/mise-action --state open --json number,author,labels,updatedAt,statusCheckRollup --limit 100 | \
+          jq -r '.[] | select(
+            (.updatedAt | fromdateiso8601) < (now - 30*24*60*60) and
+            .author.login != "jdx" and
+            ([.labels[].name] | index("keep-open") | not)
+          ) | [.number, (if (.statusCheckRollup | length > 0) and ([.statusCheckRollup[].conclusion] | index("FAILURE") or index("failure")) then "failing" else "passing" end)] | @tsv' | \
+          while read -r pr status; do
+            echo "Closing PR #$pr (checks: $status)"
+            if [ "$status" = "failing" ]; then
+              gh pr close "$pr" -R jdx/mise-action -c "This PR has been open for more than 30 days without activity. Note: CI checks were failing, which may be why it wasn't reviewed. Feel free to reopen or create a new PR if you'd like to continue working on this."
+            else
+              gh pr close "$pr" -R jdx/mise-action -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