feat(KB-093): add PR-first merge mode with configurable merge strategies

- Add mergeStrategy setting (fast-forward, squash, merge-commit) to config
- Implement PR-first auto-completion flow that monitors PR merge status
- Wire PR monitoring service to detect merge completion and trigger auto-close
- Add PR status UI to dashboard with merge progress indicator
- Update settings modal with merge strategy selector
- Add changeset for PR-first merge mode feature

packages/engine/src/pr-comment-handler.ts

@@ -128,6 +128,9 @@ export class PrCommentHandler {
 
     lines.push(`**PR Review Feedback** from @${comment.user.login}`);
     lines.push(`**PR:** #${prInfo.number} (${prInfo.status})`);
+    if (prInfo.status !== "open") {
+      lines.push(`**Note:** This PR is already ${prInfo.status}. Treat the feedback as follow-up work.`);
+    }
     lines.push("");
 
     // Truncate comment body if too long