feat: announce autonomous turn ends via _session/turn_ended

[wingleeio]

Problem

An autonomous cycle — a task-notification followup, or a peer/coordinator/observer message the model handled on its own — streams its output as out-of-turn session/update frames, but its turn-end has nowhere to go: there is no session/prompt response for it to ride, so the SDK's result message is consumed silently by the autonomous-result lane.

A client can then only guess at the turn boundary from stream silence. In practice it shows the session as "working" for however long its own silence watchdog waits after the agent already finished — in comet's case, two minutes of phantom "working" after every background-task notification (that client's report and root-cause trace: the adapter's autonomous lane breaks without emitting; journal shows output with no terminal event until the next user prompt).

Change

Emit a _session/turn_ended extension notification (_-prefixed per the ACP extension conventions, like _session/steering) from the autonomous-result lane:

• Payload: sessionId, the result's own stop_reason (the accumulated stopReason belongs to the user-turn lifecycle and may still hold the previous turn's value at this point), and the origin under _meta["_claude/origin"].
• Only between turns: sent only when no user turn is active or queued — a live turn's end rides its own prompt response, and a deferred-settle hold consumes its followup's result instead. This reuses the exact gate the branch already had for the emittedAssistantText reset.
• Best-effort: wrapped so a client that can't take the extension never kills the consumer loop; per spec, clients ignore unknown methods.

Tests

• An idle background wake announces exactly once, with the right session, stop reason, and origin.
• A background result consumed during a live prompt does not announce.
• Full suite passes (704 tests).

Verified end-to-end against a patched comet client: background sleep 8 task → prompt turn settles → wake streams "finished" → _session/turn_ended → client status settles immediately (previously: a 120s watchdog was the only settle path).