feat(acp): add --multiple-event-handling flag with cancel+re-prompt

[tlongwell-block]

Summary

Add --multiple-event-handling queue|interrupt|owner-interrupt — a single flag controlling how new @mentions are handled while a turn is already in-flight.

Mode	Behavior
queue (default)	Existing behavior. Zero code change in this path.
interrupt	Cancel in-flight turn + merged re-prompt on ANY new @mention
owner-interrupt	Cancel only for agent owner's @mentions; everyone else queues

Also adds !cancel explicit command (owner-only, mode-independent, mirrors !shutdown pattern).

How it works

New @mention → queue.push() [ALWAYS] → (mode gate) → cancel_in_flight_task()
→ select! cancel branch → cancel_with_cleanup() → PromptOutcome::Cancelled
→ requeue_as_cancelled → merged re-prompt with annotated sections

Cancel mechanics:

• Signal: tokio::sync::oneshot per channel task (None for heartbeats)
• select! { biased } prefers natural completion over cancel
• has_in_flight_prompt() guard prevents Race 1 deadlock
• Session invalidated after cancel → fresh session on re-prompt
• requeue_as_cancelled() stores events separately from generic queue
• flush_next() merges cancelled_batches into FlushBatch.cancelled_events
• format_prompt() produces annotated [Previous request — interrupted] / [New request — supersedes previous] sections
• Fallback path handles !cancel with no new events (re-dispatch unchanged)

Startup validation: Rejects interrupt/owner-interrupt with DedupMode::Drop (events would be lost during cancel drain).

Files changed

File	Lines	What
acp.rs	+5	has_in_flight_prompt() accessor
config.rs	+43	MultipleEventHandling enum, CLI arg, Config field, validation, summary
main.rs	+112	!cancel command, mode gate, cancel_in_flight_task(), oneshot dispatch, requeue_as_cancelled for Cancelled
pool.rs	+123	TaskMeta.cancel_tx, PromptOutcome::Cancelled, select! cancel logic
queue.rs	+258	FlushBatch.cancelled_events, cancelled_batches HashMap, merge logic, 5 new unit tests

Testing

• 263 tests passed (258 existing + 5 new), 0 failed
• cargo clippy clean
• New tests cover: cancel→merge flow, no-new-events fallback, has_flushable_work, drain_channel cleanup, double-cancel accumulation

Accepted limitations

• !cancel + DedupMode::Drop loses the batch (Drop mode semantics — events are expendable)
• Cancel during pre-prompt phase has latency = pre-prompt time + drain time
• Partial output notification (agent posted before cancel) deferred to UX review