fix(subagent): address codex review round 3

- Make host-user authority unforgeable. `{ kind: 'user' }` was a bare
  discriminant, so any plugin holding `ctx.subagents` — including
  model-generated cordis_mount code, which the advanced ACP composition ships
  alongside continuable subagents — could construct it and skip the
  direct-parent check for any known child id. It now carries an opaque grant
  that only SubagentService.userAuthority() mints, which composition hands to
  trusted host adapters; a model-facing tool uses parent authority from its own
  execution context.
- Reconcile a delivery discarded inside its own admission window. An enqueue
  listener that cancels fires the discard before followup() returns, so the
  discard listener could not clear an id it had not seen; submit() retained it
  and residency stayed `running` until an explicit drain.
- Recheck the caller signal after materialization. An abort landing between
  publication and inbox acceptance still submitted the prompt and returned both
  ids; it now rolls the child back.
- Stop promising the model transcript access that no shipped continuable config
  mounts. The tools now state only that a background child does not report back.
- Restate the implemented note as shipped state rather than a proposal, so it
  works as current authority.

packages/subagent/subagent/README.md

@@ -31,12 +31,13 @@ Multiple providers may coexist under different names. This lets a deployment exp
 | `start(name, request)` | Validate an ordinary caller request, then await the provider until a real one-shot child is ready. Fulfillment returns a holder-owned `SubagentRun`; rejection means the provider has already cleaned every partial startup resource. Continuable children never enter through this operation. |
 | `startContinuable(spec)` | Establish one durable continuable child and deliver its initial prompt. Resolves with `{ childId, messageId }` when the child's inbox accepts that prompt, without waiting for the turn to start or for the message to reach the Session log; any earlier failure rejects with no ids and rolls the child back entirely. Requires `ctx.agents`, session persistence, and a provider with the `prepareContinuable` capability. |
 | `followup(authority, childId, content, { source, signal })` | Deliver one later message to a continuable child as its next FIFO turn, matching `Agent.followup()` terminology, and return the accepted `AgentMessageId`. A resident child's inbox accepts it directly (waking a `waiting` Activation); an absent one cold-resumes from its persisted Session. Requires `ctx.agents`; cold resume also requires session persistence. |
+| `userAuthority()` | Mint the host-user authority a trusted adapter passes to `followup()`. Composition hands this only to a host carrying real human interaction; a model-facing tool uses its own `{ kind: 'parent', agent }` instead. |
 | `activationState(childId)` | Read one durable child's live residency state (`running`, `waiting`, or `settled`), or `undefined` when no Activation is live. |
 | `drainContinuable()` | Close continuable admission synchronously, then dispose every live Activation forest child-first. A host calls this before disposing top-level agents so no descendant outlives the runtime that owns its teardown. An aggregate error surfaces after every branch settles when any failed. |
 
 `SubagentStartRequest.signal` is required and is the canonical cancellation channel for a one-shot `start`. An abort before publication makes `start()` reject after rollback; an abort after publication cancels the live child. The request may also select a model, require structured output, cap delegation depth, restrict child tools, or set a child persona. For a continuable start or follow-up, the caller signal owns lookup, materialization, and admission only until inbox acceptance; afterward the manager owns the Activation independently, so later caller cancellation neither cancels the accepted turn nor disposes the child.
 
-Authority for continuable operations comes from a trusted host interaction or an exact live Agent tool context: `SubagentAuthority` is `{ kind: 'parent', agent }` or `{ kind: 'user' }`. The `source` on a follow-up is durable provenance retained on the delivered message and grants no authority. Parent authority requires the exact live direct parent recorded in the child's durable header; user authority may continue any child, and may cold-resume it without loading its historical parent.
+Authority for continuable operations comes from a trusted host interaction or an exact live Agent tool context: `SubagentAuthority` is `{ kind: 'parent', agent }` or `{ kind: 'user', grant }`, whose grant only `userAuthority()` mints so the discriminant alone cannot claim it. The `source` on a follow-up is durable provenance retained on the delivered message and grants no authority. Parent authority requires the exact live direct parent recorded in the child's durable header; user authority may continue any child, and may cold-resume it without loading its historical parent.
 
 Same-process requests, descriptors, results, and event payloads are trusted typed values borrowed as immutable. The service does not clone or freeze them; serialization and hostile-input validation belong at actual process, worker, persistence, and model boundaries.