ci: add stale bot for inactive issues and PRs (#228)

Closes #162.

Daily cron marks issues stale at 60d (close at 74d) and PRs stale at
30d (close at 37d). Exempt labels: `keep` for both; issues also exempt
`good first issue` and `bug`. Issues close with reason `not_planned`.

Self-contained per-repo workflow (decided over a reusable in
opendecree/.github) so each project can tune independently. The `keep`
label has been pre-created in all five repos.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: zeevdr

.github/workflows/stale.yml

@@ -0,0 +1,47 @@
+name: Stale issues and PRs
+
+# Daily sweep: marks inactive issues/PRs as stale and eventually closes
+# them. The `keep` label opts an item out entirely; `good first issue`
+# and `bug` are also exempt for issues. Issues #162 (decree).
+#
+# Tuning happens here per-repo so each project can adjust independently
+# if traffic patterns diverge.
+
+on:
+  schedule:
+    - cron: '30 1 * * *'  # daily 01:30 UTC
+  workflow_dispatch:
+
+permissions:
+  issues: write
+  pull-requests: write
+
+jobs:
+  stale:
+    runs-on: ubuntu-latest
+    timeout-minutes: 10
+    steps:
+      - uses: actions/stale@v9
+        with:
+          stale-issue-message: >
+            This issue has been inactive for 60 days and is being marked
+            stale. It will be closed in 14 days unless there is new
+            activity. Add the `keep` label to opt out of future sweeps.
+          stale-pr-message: >
+            This PR has been inactive for 30 days and is being marked
+            stale. It will be closed in 7 days unless there is new
+            activity. Add the `keep` label to opt out of future sweeps.
+          close-issue-message: >
+            Closing for inactivity. Reopen if this is still relevant.
+          close-pr-message: >
+            Closing for inactivity. Reopen if this is still relevant.
+          stale-issue-label: stale
+          stale-pr-label: stale
+          days-before-issue-stale: 60
+          days-before-issue-close: 14
+          days-before-pr-stale: 30
+          days-before-pr-close: 7
+          exempt-issue-labels: 'keep,good first issue,bug'
+          exempt-pr-labels: 'keep'
+          close-issue-reason: not_planned
+          operations-per-run: 100