fix(acp): recover omitted channel replies

[j3h7sx]

Summary

• capture final ACP assistant messages by messageId
• check for an agent-authored channel message at successful turn completion
• publish the captured answer as a signed kind-9 event when the relay confirms that the turn produced no visible message
• preserve flat top-level DMs and the triggering reply context for existing threads

Root cause

ACP sends generated assistant text through agent_message_chunk updates. Buzz Desktop shows those updates in Activity. The session/prompt result contains only a stop reason.

External ACP runtimes can finish a turn without calling buzz messages send. When that happens, the answer exists in Activity but no channel event exists for desktop or mobile clients to render.

This patch adds a harness-level delivery safety net at end_turn. An explicit agent send still wins. A relay query error or timeout skips recovery to protect against duplicate messages.

The same recovery path covers the race where a control signal arrives after the ACP prompt has already completed.

Related reports

This addresses the missing-event failure mode reported in #5388.

It complements #5635, which handles a separate desktop Inbox case where a signed NIP-10 reply exists on the relay but the DM projection omits it. The two patches have no file overlap.

Validation

• cargo test -p buzz-acp with 743 library tests and 9 lifecycle tests passing
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• cargo fmt -- --check
• git diff --check