Fix healthcheck to verify PID and add DM-only conversational mode

- 6a: Change healthcheck in Dockerfile and docker-compose.yml to verify
  the PID file exists and the process is alive (kill -0) instead of
  testing SQLite connectivity, which only proves the DB file exists
- 6b: In DMs, skip !commands so MeshMonitor or other bots handle them.
  MeshAI only responds conversationally in DMs (no bang commands)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

meshai/router.py

@@ -80,12 +80,11 @@ class MessageRouter:
         if message.sender_id == self.connector.my_node_id:
             return False
 
-        # Check if DM
+        # Check if DM — conversational mode only, skip !commands
+        # (let MeshMonitor or other bots handle bang commands in DMs)
         if message.is_dm:
-            # In DMs, let commands through to dispatcher but skip !commands
-            # that should be handled by other bots (like MeshMonitor)
             if self.dispatcher.is_command(message.text):
-                return True
+                return False
             return self.config.bot.respond_to_dms
 
         # Check channel filtering