feat(FN-840): add closed/merged PR feedback contract and clean up unused code

- Add closed/merged PR feedback contract to pr-monitor for post-merge follow-up behavior
- Fix PR monitor drainComments + consume/drain behavior with comprehensive tests
- Add PR feedback follow-up tests and scheduler follow-up logic
- Remove unused activity log, agent log viewer, and multi-agent log code from dashboard
- Remove unused store methods, types, API endpoints, and route handlers
- Update README docs to document manual-merge PR follow-up behavior and fix auth wording
- Add dashboard CLI test coverage for new commands

packages/cli/src/commands/dashboard.ts

@@ -606,6 +606,9 @@ export async function runDashboard(port: number, opts: { paused?: boolean; dev?:
       prMonitor,
       onSchedule: (t) => console.log(`[engine] Scheduled ${t.id}`),
       onBlocked: (t, deps) => console.log(`[engine] ${t.id} blocked by ${deps.join(", ")}`),
+      onClosedPrFeedback: async (taskId, prInfo, comments) => {
+        await prCommentHandler.createFollowUpTask(taskId, prInfo, comments);
+      },
     });
 
     // ── CronRunner: scheduled task execution ──────────────────────────