subagent: inherit parent sandbox/approval overrides in in-process children

Per-session policy overrides (sandbox/mode, approval/policy) never crossed
the delegation boundary: a spawn child of a read-only-switched parent ran
under the wider deployment default, and a fork child missed any switch made
after its seed boundary — delegation was a bypass channel for a user's
tightening.

The in-process driver now snapshots the delegating parent's override chain
and stamps it onto the child through the canonical write paths
(SandboxPolicyService.inheritOverride / ApprovalService.inheritOverride),
anchored inside the child's first turn via a one-shot agent/prompt-submit
listener: turn-enclosed (durable), ahead of the first request (an inherited
'never' reaches the child's first system prompt), and positioned after any
stale fork-seed switch so the ordinary last-event-wins fold resolves it.
Only overrides are copied — an unswitched parent stamps nothing and the
child follows the live deployment default; both services are consumed
opportunistically, so compositions without them delegate unchanged. Nesting
composes by construction (each stamp folds the already-stamped parent log).

Evidence: inheritance.spec.ts drives scripted-model children into the real
dsh-fs-sandbox fence through the real write tool (disk-state + denial-marker
assertions; spawn, stale-seed fork, grandchild, escalation fail-closed, and
no-stamp guards), inheritOverride contract tests in both service suites, and
the recorded subagent-sandbox-inheritance ACP snapshot (read-only preset →
delegate → child denied, replayed keylessly).

See .agents/notes/implemented/feature/2026-07-25-subagent-policy-inheritance.md.

packages/ui/user-approval/tests/approval.spec.ts

@@ -576,3 +576,46 @@ describe('approval policy (the approval/policy fold)', () => {
     expect(afterDispose.injected).toEqual([])
   })
 })
+
+describe('inheritOverride (parent → child stamping)', () => {
+  const policyEvents = (session: Session) => session.events.filter(e => e.type === 'approval/policy')
+
+  function bareSession(id: string): Session {
+    return new Session(SessionId(id))
+  }
+
+  it('stamps the parent LAST override onto the child through the canonical write path', async () => {
+    const ctx = await mounted()
+    const parent = bareSession('sess-appr-inherit-parent')
+    const child = bareSession('sess-appr-inherit-child')
+    setApprovalPolicy(parent, 'never')
+
+    ctx.approval.inheritOverride(parent, child)
+
+    const stamped = policyEvents(child)
+    expect(stamped).toHaveLength(1)
+    expect(stamped[0]?.data).toEqual({ policy: 'never' })
+  })
+
+  it('appends NOTHING when the parent never switched (the configured default must stay live)', async () => {
+    const ctx = await mounted()
+    const parent = bareSession('sess-appr-default-parent')
+    const child = bareSession('sess-appr-default-child')
+
+    ctx.approval.inheritOverride(parent, child)
+
+    expect(child.events).toHaveLength(0)
+  })
+
+  it('skips the append when the child already folds to the inherited policy (fork-seed dedup)', async () => {
+    const ctx = await mounted()
+    const parent = bareSession('sess-appr-dedup-parent')
+    const child = bareSession('sess-appr-dedup-child')
+    setApprovalPolicy(parent, 'never')
+    setApprovalPolicy(child, 'never')
+
+    ctx.approval.inheritOverride(parent, child)
+
+    expect(policyEvents(child)).toHaveLength(1)
+  })
+})