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/tool-subagent/src/index.ts

@@ -207,8 +207,8 @@ export function apply(ctx: Context, config: Config): void {
       description: wording.description + (backgroundEnabled
         ? continuable
           ? ' Set `run_in_background: true` to start a background subagent that keeps its conversation:'
-          + ' you receive its subagent id and it works on its own. It does not report back to you, so read'
-          + ' its transcript by that id, or send it more work with `send_message`.'
+          + ' you receive its subagent id and it works on its own. It does not report back, so use this'
+          + ' only for work whose result you do not need returned; `send_message` sends it more work.'
           : ' Set `run_in_background: true` to return a task id; collect with `task_output` and stop with `task_kill`.'
         : ''),
       parameters: {
@@ -226,8 +226,8 @@ export function apply(ctx: Context, config: Config): void {
           run_in_background: {
             type: 'boolean' as const,
             description: continuable
-              ? 'Run as a background subagent that keeps its conversation and return its subagent id; '
-              + 'send it more work with send_message.'
+              ? 'Run as a background subagent that keeps its conversation and return its subagent id. '
+              + 'It does not report its result back; send it more work with send_message.'
               : 'Run as a background task and return its id; collect with task_output or stop with task_kill.',
           },
         } : {},