ci: let stale bot close PRs waiting for reply

The workflow was missing the pull-requests write permission, so its PR
settings never had any effect. Grant it and close PRs two weeks after
the author goes silent, matching the contribution policy.

.github/workflows/stale-waiting-for-reply.yml

@@ -1,4 +1,4 @@
-name: Close stale "waiting for reply" issues
+name: Close stale "waiting for reply" issues and PRs
 
 on:
   schedule:
@@ -7,6 +7,7 @@ on:
 
 permissions:
   issues: write
+  pull-requests: write
 
 jobs:
   stale:
@@ -26,5 +27,9 @@ jobs:
             for the report!
           stale-pr-label: 'waiting for reply'
           days-before-pr-stale: 30
-          days-before-pr-close: -1
+          days-before-pr-close: 14
+          close-pr-message: >
+            Closing this since we haven't heard back on the open review
+            questions. If you're still interested in landing this, answer
+            them in a comment and we'll reopen.
           operations-per-run: 100