Finish the session: drain, checkpoint, close for good

Ends a session whose work is done. Unlike terminate, nothing is killed: the running turn finishes, its checkpoint commits, and the session ends closed, which nothing reopens. Unlike a recovery close (POST /v1/sessions/{id}/recovery-decisions, decision: close), it is the owner's call on a healthy session, needs sessions:control rather than sessions:recover, and its receipt operation is close rather than recovery_decision. The API guide compares the three.

• active with a live worker: the worker drains as for a pause and the session goes closing. The receipt stays accepted until the execution is observed gone with a checkpoint covering every turn that ran; then the session is closed and the receipt succeeded. A drain over 60 seconds shows attention.code: CLOSE_BLOCKED with the reason; nothing is killed for it. An open question or approval holds the close until it is answered or the turn interrupted. If the drain cannot commit, the receipt fails and the session goes back to active (or recovery_required for a turn whose outcome is unknown), with its queued input intact; a terminate overtakes it (CONTROL_SUPERSEDED).
• active with no live worker, paused, or a stopped session that can be resumed: closed in this request. With an execution still bound the receipt is accepted until it is observed gone; otherwise succeeded in the response.

Queued input is cancelled in the transaction that closes the session (each input's receipt fails with SESSION_CLOSED). Push requests are left alone; request a push of the result after the close has succeeded, since a push requested inside the last turn is refused CHECKPOINT_MOVED once the close commits that turn's checkpoint. The receipt result is {resulting_admission_state, checkpoint_revision, cancelled_turn_count}. The stream carries a status event for closing and for closed, and a system event {subtype: session_closed, closed_by: close, close_receipt_id, checkpoint_revision, cancelled_turn_count}. A caller closing its own session from inside a turn must not wait on the receipt: the drain waits for that turn to end. After the close, messages and controls answer 409 SESSION_CLOSED.

Scope: sessions:control (API key or cookie session). Idempotency-Key: required.