Capture a retained output prefix

Explicitly freeze observed background output into a new immutable result. Retrying creates another result; it never executes the command. No automatic capture, Activities record, listing or idempotency key. Accepts an application/json object (including {}) of at most 4096 bytes. The total capture deadline is 60000 milliseconds. Capture starts at independent zero offsets, stops each stream at observed EOF or its requested cap, and stores the wrapper diagnostic once. This is a finite prefix from mutable guest paths, not an atomic snapshot or completion proof. Expiry is measured from capture_started_at. No query parameters or arbitrary metadata are accepted. A lost response may leave a committed result until expiry. Responses are no-store; Location is the relative metadata URL.

Requires the member role or higher.