fix(acp): acknowledge consumed owner commands in channel

[tcballard]

Summary

Implements the acknowledgement-only slice of #3711, as part of the ordered workstream tracked by #3863.

Adds:

• visible channel acknowledgements when an owner consumes !cancel, !rotate, or !shutdown;
• state-specific wording for active, idle, cached-session, and empty-session cases;
• focused ACP regression tests.

No desktop UI changed, so no screenshots are included.

Scope

Included

• !cancel publishes either Cancelled the current turn. or Nothing to cancel — no turn in flight.
• !rotate publishes the active-turn, cached-idle, or empty-session acknowledgement defined in Owner control commands are invisible in the chat: no command affordance, and the outcome is only logged #3711.
• !shutdown publishes Shutting down. before the shutdown signal is sent.
• Notices remain channel-scoped, threaded kind-9 events.

Excluded

• Slash-command autocomplete and command-palette work (feat(desktop): slash-command autocomplete in composer from ACP available_commands_update (follow-up to #919) #2528).
• Agent role descriptions (feat(desktop): show each agent's role/description in the @-mention selector #2699).
• Canvas revision history (Desktop: expose channel canvas revision history #3086).
• Structured job handoff cards (Make completed agent work inspectable without reading the full thread #2932).
• The native signed extension panel work described in the follow-up handoff.
• OEXL matching, payments, acceptance, ledger, settlement, or /bounty logic.
• New HTTP APIs, new Buzz event kinds, or broad UI redesign.

Product / Architecture Decisions

• Generalized the existing failure-notice publishing path from post_failure_notice to post_notice.
• Reused the existing ThreadTags and kind-9 channel-notice path used by failure notices and the owner !model acknowledgement path.
• Shutdown awaits the notice publish attempt before sending the shutdown signal; cancel and rotate use the existing best-effort spawned notice pattern.
• Buzz remains responsible for channel visibility and auditability. No OEXL-specific protocol or settlement semantics were added.

User-Facing Contract

Command	Situation	Notice
!rotate	Turn active	Cancelled the current turn — the next one starts from a fresh session.
!rotate	Cached session, idle	Session rotated — the next turn starts fresh.
!rotate	No cached session	No session to rotate — the next turn already starts fresh.
!cancel	Turn active	Cancelled the current turn.
!cancel	Idle	Nothing to cancel — no turn in flight.
!shutdown	Any	Shutting down.

Verification

Ran

• cargo test -p buzz-acp owner_control_command_tests — 6 passed.
• rustfmt --edition 2021 crates/buzz-acp/src/lib.rs crates/buzz-acp/src/pool.rs --check — passed.
• cargo clippy -p buzz-acp --all-targets -- -D warnings — passed.
• ./scripts/run-tests.sh unit — buzz-core (237 passed) and buzz-auth (45 passed) completed before the broader run was stopped during a local buzz-cli compile stall.

Not completed locally

• The whole-workspace just fmt-check / just clippy / just test-unit / just ci sequence was not completed because the first-run workspace formatter/toolchain stalled in the local environment. The focused ACP gates above passed; CI should provide the full repository gate.

Covered

• Active and idle !cancel.
• Active, cached-idle, and empty !rotate.
• !shutdown acknowledgement.
• Notice text and helper delegation remain isolated from desktop/mobile UI.

Review Path

1. crates/buzz-acp/src/pool.rs notice helper generalization.
2. crates/buzz-acp/src/lib.rs owner-command acknowledgement sequencing.
3. Focused owner-control regression tests.

Notes For Reviewer

This is intentionally the smallest channel-notice contribution for #3711 and the first ordered contribution in #3863. Command-palette/autocomplete presentation remains deferred to its separate UI work. No desktop UI changed, and no OEXL-specific code or settlement semantics were added.