Solves the "ships passing in the night" problem where agents publish
messages before other agents have subscribed, causing dropped messages.
New tools:
- mqtt_host_conversation: Initiating agent hosts and waits for joiners
- mqtt_join_conversation: Joining agents connect and signal ready
- mqtt_conversation_status: Check conversation state
- mqtt_list_conversations: List active conversations
The protocol guarantees no messages are dropped by ensuring all expected
agents are subscribed before the host begins publishing.
