feat(acp): steer claude-code and codex agents via _session/steering

[wpfleger96]

Mid-turn steering was reachable only through goose's _goose/unstable/session/steer, which requires an expectedRunId sourced from _meta.goose.activeRunId. claude-agent-acp and codex-acp never emit a run id, so every mid-turn mention to those harnesses bailed at the run-id guard before writing a byte and degraded to cancel + merge, destroying in-flight tool calls.

Both adapters ship _session/steering (params {sessionId, prompt}, result {outcome}) and advertise it as _meta.steering.supported on the initialize response. This adds it as a second steer transport selected at write time, reusing the existing withhold/release, ack routing, and cancel+merge fallback machinery unchanged.

Transport selection

active_run_id	steering_supported	Transport
Some(run_id)	any	_goose/unstable/session/steer + expectedRunId (unchanged)
None	true	_session/steering with {sessionId, prompt}
None	false	ack ExpectedRunIdMissing, write nothing (unchanged)

goose keeps priority when both are present — expectedRunId is strictly more precise about which run is being steered.

Two load-bearing safety properties

The advertised capability is the only gate — never error-code probing. codex-acp's extMethod answers unrecognized extension methods with a bare {}, which is a JSON-RPC success rather than -32601. Buzz maps a steer success to queue.remove_event, so probing an unknown method would silently delete the user's message with no error, no fallback, and no log line.

An outcome must be positively recognized. Only injected and startedNewTurn count as delivery. Anything else — codex's failed, an unknown value, or a missing outcome entirely — is SteerError::OutcomeRejected, which releases the withheld event and fires the cancel+merge fallback. This makes the silent-loss path above unreachable even if an adapter mis-advertises.

startedNewTurn acks Success, because the message really was delivered and must not be redelivered, but deliberately does not renew the read loop's hard deadline: the turn Buzz was awaiting had already settled, and renewing would extend the clock on a finished turn.

Notes for reviewers

• SteerError::OutcomeRejected needs no new arm in the PoolEvent::SteerAck match — the existing catch-all Ok(SteerAck::Err(_)) => (true, false, true) already gives release + fallback, and the two AgentError arms above it match that variant specifically, so they do not shadow it.
• Comments that described the old goose-only "try-and-tolerate" -32601 behavior are corrected; that assumption was never valid for codex-acp.
• No CI job runs buzz-acp tests. The full package suite was run locally: 617 passing, 0 failing.