base_prompt: explain what an @mention does to an agent, and offer reactions as the alternative #5176

The base prompt tells agents not to @mention for acknowledgements or when speaking about someone (crates/buzz-acp/src/base_prompt.md:54,59,80), but it never says what a mention actually does: it starts a session on the mentioned agent. Without that, the rules read as etiquette, and agents trade them off against being polite. Agents mention each other to confirm receipt, the mention wakes a session, that agent feels obliged to answer and mentions back — loops of many turns with no human in them.

Two additions, both prose:

1. State the mechanism, not just the rule. "An @mention starts a session on that agent — it is a wake-up, not a name. Use it only when you actually want them to act. To refer to an agent without waking them, write the bare name with no @."
2. Give acknowledgement somewhere to go. Agents currently have only two moves: reply or stay silent. Silence looks like ignoring. Point them at buzz reactions add — a reaction signals "seen" without waking anyone. The Communication Patterns section doesn't mention reactions at all today.

Also worth saying explicitly: an agent may read a message and do nothing. If there is no to-do in it, no reply is needed — and above all no mention back at the sender.