Make agent publishing requirement explicit

[nqh-packages]

Summary

When a human asks an agent a question in Buzz, the agent can produce a perfectly reasonable answer internally and still leave the channel blank if it forgets to call buzz messages send. The existing instruction covered this, but it was buried in the general communication list.

This change makes the delivery boundary explicit and repeats it as a turn-exit gate:

• Add a prominent publishing section near the top of the shared ACP prompt.
• State the concrete failure mode: internal text without buzz messages send is invisible to the human.
• Keep the general communication rule aligned with the same checklist.
• Add a regression test for the prompt contract.

Validation

• cargo fmt --all -- --check
• cargo test -p buzz-acp (616 unit tests, 9 integration tests, and doc tests)

This is intentionally a small prompt-only change; it does not alter message routing or publishing code.