fix(buzz): discover joined rooms and support room-scoped free response

[coldcrippy]

Summary

• preserve upstream joined-channel membership discovery from fix(buzz): discover newly joined channels dynamically #74823 (original commit credited to SHL0MS)
• use one discovery path for WebSocket membership events and periodic polling
• seed each newly discovered room/DM before dispatch and reject pre-discovery events
• support canonical free_response_channels for room-scoped mention bypass while preserving self-echo and author allowlists

Verification

• scripts/run_tests.sh tests/gateway/test_buzz_adapter.py tests/gateway/test_buzz_websocket.py -q — 81 passed
• ruff check + git diff --check — clean
• independent immutable-candidate review on a5851b1c2c0f3c09706a19ca279da93967f6355e — CLEAN

[ScaleLeanChris]

Tested current head a5851b1c2c0f3c09706a19ca279da93967f6355e in an isolated live Hermes gateway against a real Buzz relay over the WebSocket transport.

Automated check:

• pytest tests/gateway/test_buzz_adapter.py -q: 34 passed
• No source or test modifications were made.

Live acceptance:

1. Joined-room discovery: a room joined after gateway startup produced subscribed to new conversation, and the adapter began receiving live events without a restart.
2. Pre-discovery boundary: an addressed message created before the agent joined was seeded but never dispatched or answered after discovery.
3. Room-scoped free response: an unmentioned message in a configured free_response_channels room was dispatched and received the requested reply.
4. Ordinary mention gating: an unmentioned message in a normal room was not dispatched and received no reply; the following explicit @Fly message was dispatched and answered.
5. Authorization remained independent: the live probes required the sender to pass the existing user allowlist. The free-response exemption did not bypass that gate.

Temporary room and authorization settings were restored after the run. The candidate gateway reconnected cleanly afterward.

The discovery handoff worked “Just in Time”: history stayed behind the cursor, while the first live message after subscription was handled.