test(subagent): drop the vacuous fork-dedup case; align wording with the header baseline

The dedup guard pinned 'no duplicate stamp' — under the header baseline no
stamping exists, so its event-count assertion became a tautology and its
fence assertions a subset of the stale-seed case. The spec docblock claimed
the session invariants pin the stamping design (superseded); it now names
the three review-found timing threats the suite pins as distinct shapes.
Service overrideOf JSDoc shrinks to a pointer at the owning pure export.

packages/sandbox/sandbox-policy/src/index.ts

@@ -106,19 +106,12 @@ export class SandboxPolicyService extends Service {
   }
 
   /**
-   * A session's sandbox-mode OVERRIDE — the override chain alone, never the
-   * deployment default: the fold of the session's OWN switches (events past
-   * the seed boundary — a fork seed's stale parent switch is subsumed by the
-   * baseline captured after it), else the header's inherited delegation
-   * baseline. The subagent driver stamps `overrideOf(parent.session)` into
-   * each child's creation meta, so the chain collapses one level per
-   * delegation and a tightened parent binds children at any depth
-   * ([rationale](../../../.agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md)).
+   * {@link sandboxOverrideOf} surfaced on the service, for consumers that
+   * reach policy through `ctx.get('sandboxPolicy')` (the subagent driver's
+   * delegation capture, pty-local) rather than a value import.
    * @param session - the session whose override chain to resolve.
    * @returns the effective override, or `undefined` for a session following
    *   the deployment default.
-   * @throws when the durable header baseline is outside the closed mode
-   *   vocabulary (a corrupt or foreign log; durable-boundary validation).
    */
   overrideOf(session: Session): SandboxMode | undefined {
     return sandboxOverrideOf(session)