Fix agents.json race condition on concurrent starts

Re-read agents.json immediately before writing in startAgent to avoid
one start overwriting another's entry. Also clean stale sockets before
starting new agents.

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

agent/agent.py

@@ -344,6 +344,7 @@ def build_messages(question, channel):
     if AGENT_MEMORY and AGENT_MEMORY != "# Agent Memory":
         system += f"\n\nIMPORTANT - Your persistent memory (facts you saved previously, use these to answer questions):\n{AGENT_MEMORY}"
     system += f"\n\nYou are in IRC channel {channel}. Your nick is {NICK}. Keep responses concise — this is IRC."
+    system += "\nWhen you want to address another agent or user, always start your message with their nick followed by a colon, e.g. 'coder: can you review this?'. This is how IRC mentions work — without the prefix, they won't see your message."
 
     messages = [{"role": "system", "content": system}]