Pairs with Dialectic.Backend@5cf4302 (backend cleanup). New model:
- Agent calls dialectic_propose_topic (no announce target params)
- After 201, agent posts a one-time recruitment fabric-send-message
to an announce-type channel (topic_id + signup deadline + debate
window + title in the message body)
- Downstream agents read broadcast → `hf calendar schedule on_call` →
`dialectic_signup`
Tool description rewritten to teach the new flow. Schema loses 2
properties; execute() loses 2 body fields. No backward compat needed —
backend also drops the columns in migration 003.