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/src/index.ts

@@ -54,6 +54,7 @@ import SubagentContinuationManager from './continuation.ts'
 import type {
   ActivationObserver,
   ActivationState,
+  UserAuthorityGrant,
   ContinuableStart,
   ContinuableStartSpec,
   SubagentAuthority,
@@ -94,6 +95,7 @@ export type { ChildComposition } from './child-agent.ts'
 export type {
   ActivationObserver,
   ActivationState,
+  UserAuthorityGrant,
   ContinuableStart,
   ContinuableStartSpec,
   CoordinatorMessageSource,
@@ -174,6 +176,15 @@ export interface SubagentRunEndInfo {
 export class SubagentService extends Service {
   private providers = new Map<string, SubagentProvider>()
   private continuations: SubagentContinuationManager | undefined
+  /**
+   * The process-local proof of host-user authority. Minted here so the value is
+   * unguessable and unforgeable: a caller must obtain it from
+   * {@link userAuthority}, which composition hands only to trusted host
+   * adapters.
+   */
+  private readonly userGrant = Object.freeze({
+    __brand: 'SubagentUserAuthority',
+  }) as UserAuthorityGrant
 
   constructor(ctx: Context) {
     super(ctx, 'subagents')
@@ -181,7 +192,7 @@ export class SubagentService extends Service {
       const manager = new SubagentContinuationManager(childCtx, {
         prepareContinuable: (name, request) => this.prepareContinuable(name, request),
         observeActivation: (provider, childId, parent) => this.observeActivation(provider, childId, parent),
-      })
+      }, this.userGrant)
       this.continuations = manager
       childCtx.effect(() => () => {
         /* v8 ignore else -- one injected binding owns the slot until its fiber disposes. */
@@ -227,6 +238,17 @@ export class SubagentService extends Service {
     return this.requireContinuations().followup(authority, childId, content, options)
   }
 
+  /**
+   * Host-user authority for continuable operations, which may continue any
+   * durable child without its parent. A composition passes this only to a
+   * trusted host adapter carrying real human interaction; a model-facing tool
+   * uses `{ kind: 'parent', agent }` from its own execution context instead.
+   * @returns the authority a host adapter supplies to {@link followup}.
+   */
+  userAuthority(): SubagentAuthority {
+    return { kind: 'user', grant: this.userGrant }
+  }
+
   /**
    * Read one durable child's live residency state.
    * @param childId - durable child session id.