Add thread safety and agent health checks

- IRC socket writes protected by threading.Lock in agent.py
- Overseer runs periodic health check (30s interval)
- Detects dead agent processes, cleans up resources, announces in #agents

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

agent/agent.py

@@ -118,6 +118,7 @@ class IRCClient:
         self.nick = nick
         self.sock = None
         self.buf = ""
+        self._lock = threading.Lock()
 
     def connect(self):
         self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
@@ -127,7 +128,8 @@ class IRCClient:
         self.send(f"USER {self.nick} 0 * :Fireclaw Agent")
 
     def send(self, msg):
-        self.sock.sendall(f"{msg}\r\n".encode("utf-8"))
+        with self._lock:
+            self.sock.sendall(f"{msg}\r\n".encode("utf-8"))
 
     def join(self, channel):
         self.send(f"JOIN {channel}")