Wire heartbeat into NexusMind consciousness loop

The heartbeat module existed but was never called. Now write_heartbeat fires:
- On startup (cycle 0, status thinking)
- After every successful think cycle
- On graceful shutdown (status idle)

This gives the watchdog a signal that the mind is alive, not just running.

nexus/nexus_think.py

@@ -45,6 +45,7 @@ from nexus.perception_adapter import (
 )
 from nexus.experience_store import ExperienceStore
 from nexus.groq_worker import GroqWorker
+from nexus.heartbeat import write_heartbeat
 from nexus.trajectory_logger import TrajectoryLogger
 
 logging.basicConfig(
@@ -286,6 +287,13 @@ class NexusMind:
 
         self.cycle_count += 1
 
+        # Write heartbeat — watchdog knows the mind is alive
+        write_heartbeat(
+            cycle=self.cycle_count,
+            model=self.model,
+            status="thinking",
+        )
+
         # Periodically distill old memories
         if self.cycle_count % 50 == 0 and self.cycle_count > 0:
             await self._distill_memories()
@@ -383,6 +391,13 @@ class NexusMind:
             salience=1.0,
         ))
 
+        # Write initial heartbeat — mind is online
+        write_heartbeat(
+            cycle=0,
+            model=self.model,
+            status="thinking",
+        )
+
         while self.running:
             try:
                 await self.think_once()
@@ -423,6 +438,13 @@ class NexusMind:
         log.info("Nexus Mind shutting down...")
         self.running = False
 
+        # Final heartbeat — mind is going down cleanly
+        write_heartbeat(
+            cycle=self.cycle_count,
+            model=self.model,
+            status="idle",
+        )
+
         # Final stats
         stats = self.trajectory_logger.get_session_stats()
         log.info(f"Session stats: {json.dumps(stats, indent=2)}")