Submit a prune intent (owner)

Publishes the owner-signed prune_intent. Everything fallible happens before the intent's pre-I/O fence; once that transaction commits nothing refuses and the platform worker replays the remaining steps identically, so stored: false means “the create-only intent write is not yet observed”, never “this failed”. The request body is read as exact bytes — the intent is strict-parsed and signature-verified over exactly what you sent, so send it with Content-Type: application/json and do not let a client library re-serialize it.