fix(acp): deliver plain-text replies when send is skipped

[nimittamatram07-lab]

Summary

Fixes silent conversational turns from ACP runtimes that return assistant text
without explicitly calling buzz messages send.

• Buffers agent_message_chunk text for the current ACP turn.
• On a successful EndTurn, posts buffered text as a harness-signed threaded
  kind-9 reply when no publish delivery was confirmed.
• Tracks publish candidates through their terminal tool outcome, so failed or
  cancelled sends leave the fallback armed while successful sends suppress it
  and avoid duplicates.
• Does not post partial output from truncated/aborted turns, heartbeats without
  a reply target, or bare acknowledgements.
• Keeps signing credentials in buzz-acp; the child runtime never needs the
  Buzz private key.

This rebases and adapts the approach from #2681 onto current main, preserving
the original contributors in the commit trailers and resolving the intervening
turn-lifecycle changes.

Related issue

Closes #2698.

Related implementation: #2681.

Testing

Passed:

• cargo fmt --all -- --check
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• cargo test -p buzz-acp — 668 passed, 0 failed
• cargo build --release -p buzz-acp
• Full just ci stages through:
  • workspace formatting and strict Clippy
  • Desktop checks and Rust Clippy
  • Web checks
  • mobile formatting and analysis
  • Rust unit-test suite

Live end-to-end verification used an existing local Hermes ACP agent:

• Trigger event:
  a59d58941f1fa83d4029aa3032308711d8a59b7fe52878f5620e4a330dca58f9
• Harness-fallback reply:
  6cfcf56c2ee63029c90494dcc5f1cd04df0e4261648974a8f3d84964be87db29
• Reply content: Existing Hermes connected securely.
• The reply was signed by the existing agent identity, included the expected
  channel/root/reply tags, and produced no duplicate.

The repository-wide just ci run ultimately stopped in the pre-existing
Desktop scroll-test suite: multiple unchanged tests fail with
TypeError: act is not a function. This PR changes only
crates/buzz-acp/src/acp.rs and crates/buzz-acp/src/pool.rs; the focused
buzz-acp suite and all earlier repository-wide stages are green.

[Chessing234]

plain-text fallback when send is skipped is useful. does the activity log still show the skipped send so operators can tell delivery was degraded?

[nimittamatram07-lab]

@Chessing234 Good catch. The original patch emitted a harness WARN, but it did not add an explicit degraded-delivery item to the Desktop Activity feed. Fixed in ff34b9c: successful fallback delivery now emits a Delivery fallback activity item, while a fallback publish failure emits Delivery fallback failed with the reason. The full buzz-acp suite passes (668 tests), and strict Clippy is clean.