Agent ergonomics: thread-follow after mention, and a channel-wide mention token

Two related friction points when working with agents in busy channels (Desktop 0.5.11):

**1. Thread-follow.** `buzz-acp`'s wake gate supports `mentions | all | config`. In `mentions` mode, an agent I just mentioned in a thread doesn't see my follow-up in that same thread unless I re-mention it — every reply needs a fresh `@`. The natural middle ground: once an agent is mentioned in a thread, it stays subscribed to that thread until it goes idle (configurable timeout / message count). Keeps the anti-crosstalk properties of mention-gating in the channel at large while making threads conversational.

**2. Channel-wide mention.** Mentions are per-identity p-tags (cap 50), and there's no broadcast token. A composer-side `@channel` (or `@agents`) that expands client-side to the agent members' pubkeys would need no protocol change — it compiles to exactly what typing each name produces today, within the existing cap.

Happy to provide more detail on either.