feat(FN-978): add diagnostic logging, semaphore resilience, and executor tests

- Add structured diagnostic logging to executor, stuck-task-detector, and pi.ts with subsystem prefixes
- Add defensive guards to AgentSemaphore (limit minimum 1, invalid limit handling)
- Add comprehensive integration tests for agent execution flow (executor.test.ts)
- Add unit tests for semaphore resilience (concurrency.test.ts) and stuck-task-detector (stuck-task-detector.test.ts)
- Fix TypeScript errors in test task objects and duplicate execution test
- Document engine diagnostic logging points in AGENTS.md

packages/engine/src/stuck-task-detector.ts

@@ -137,6 +137,7 @@ export class StuckTaskDetector {
       lastProgressAt: now,
       activitySinceProgress: 0,
     });
+    stuckLog.log(`Tracking task ${taskId} (total tracked: ${this.tracked.size})`);
   }
 
   /**
@@ -157,6 +158,9 @@ export class StuckTaskDetector {
     if (entry) {
       entry.lastActivity = Date.now();
       entry.activitySinceProgress++;
+      if (entry.activitySinceProgress <= 3 || entry.activitySinceProgress % 50 === 0) {
+        stuckLog.log(`Activity recorded for ${taskId} (sinceProgress=${entry.activitySinceProgress})`);
+      }
     }
   }