ci: shorten stale-PR auto-close threshold from 30 to 14 days

Shorten the inactivity window before a stale PR is auto-closed from ~1 month
to 2 weeks, keeping the PR queue tidier and giving contributors quicker
feedback (they can always reopen or file a new PR). The comment body already
interpolates ${STALE_DAYS} dynamically, so only the constant and the step name
needed updating; no README/docs reference the policy.

Closes #2160

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: cm-dyoshikawa

.github/workflows/auto-close-stale-prs.yml

@@ -18,11 +18,11 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 10
     steps:
-      - name: Close pull requests inactive for more than 30 days
+      - name: Close pull requests inactive for more than 14 days
         uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
         with:
           script: |
-            const STALE_DAYS = 30;
+            const STALE_DAYS = 14;
             const staleThresholdMs = STALE_DAYS * 24 * 60 * 60 * 1000;
             const now = Date.now();
             const { owner, repo } = context.repo;