Shared conversation enhancement

[dvf]

Main changes:

• synapse_p2p/types.py
  • Added ConversationEvent.
• synapse_p2p/node.py
  • Added local conversation event log: node.conversation(...).
  • Added node.emit_conversation_event(...).
  • Added opt-in node.ack(...).
  • Added _synapse.conversation.event endpoint with dedupe + gossip.
  • Broadcasts now create a message event.
  • Replies now create reply conversation events.
  • Lifecycle hooks now support:
    • conversation.event
    • conversation.ack
    • conversation.reply
    • etc. based on event kind.
• synapse_p2p/init.py
  • Exported ConversationEvent.
• synapse_p2p/tests/test_broadcast.py
  • Added tests for message/reply event recording, ACK propagation, and event dedupe.
• README.md
  • Added docs/examples for ACK and conversation events.
• Updated CLI stuff

Important design stuff:

• ACK is opt-in, not automatic.
• Synapse does not decide who should reply.
• Nodes can wade in or stay silent.
• Higher-level agent frameworks can add their own event kinds.