subagent: carry inherited policy overrides in the child session header

Review fix (ds-review-bot critical #2 on #623): the first-turn event stamp
had a durability hole no turn anchoring can close — an idle SessionStart-
style injection persists a complete one-shot turn before any prompt turn
opens, so a crash in that window left a resumable-looking child with no
inherited policy, falling back to a possibly wider deployment default.

The captured overrides now ride the child's creation meta into its
immutable SessionHeader (sandboxMode/approvalPolicy, neutral strings at the
session boundary — the delegationDepth precedent), durable from the moment
the session exists: no listener ordering can starve the baseline and no
crash window can lose it. overrideOf(session) on both policy services
resolves fold(events past header.seedLength) ?? header baseline, validating
against the closed vocabulary on read; stampOverride and the prompt-submit
listener machinery are deleted. The header field rides both persistence
backends (JSONL header line; SQLite sessions columns, SCHEMA_VERSION 11 —
pre-release, no migration). pty-local reads through overrideOf so PTY
spawns see the baseline too.

Red-first: header-durability-before-any-turn test (the injection crash
window shape), baseline/seed-boundary/closed-vocabulary contract tests in
both service suites; the real-wall suite (race, veto, fork stale-seed,
grandchild) re-anchored on header assertions and green. The Agent Note's
Alternatives now records the superseded event-stamping iteration with the
review evidence; bilingual docs updated.

packages/sandbox/sandbox-policy/tests/policy.spec.ts

@@ -143,41 +143,56 @@ describe('the sandbox/mode session kit', () => {
   })
 })
 
-describe('delegation inheritance (overrideOf + stampOverride)', () => {
-  const modeEvents = (session: Session) => session.events.filter(e => e.type === 'sandbox/mode')
+describe('delegation inheritance (overrideOf over the header baseline)', () => {
+  /** A session whose header carries the delegation-inheritance baseline. */
+  function inheritedSession(id: string, meta: { sandboxMode?: string; seedLength?: number } = {}): Session {
+    const sessionId = SessionId(id)
+    return new Session(sessionId, undefined, {
+      version: 0,
+      id: sessionId,
+      createdAt: 0,
+      ...meta.sandboxMode === undefined ? {} : { sandboxMode: meta.sandboxMode },
+      ...meta.seedLength === undefined ? {} : { seedLength: meta.seedLength },
+    })
+  }
 
-  it('overrideOf folds to the LAST override and never falls back to the deployment default', async () => {
+  it('overrideOf folds the session log and never falls back to the deployment default', async () => {
     const ctx = await mounted({ mode: 'workspace-write' })
     const parent = session('sess-inherit-parent')
     setSandboxMode(parent, 'workspace-write')
     setSandboxMode(parent, 'read-only')
 
     expect(ctx.sandboxPolicy.overrideOf(parent)).toBe('read-only')
-    // undefined, NOT the deployment default — a child stamped with the
+    // undefined, NOT the deployment default — a child whose header froze the
     // default would stop following the LIVE default across resumes.
     expect(ctx.sandboxPolicy.overrideOf(session('sess-inherit-unswitched'))).toBeUndefined()
   })
 
-  it('stampOverride appends the captured mode through the canonical write path', async () => {
-    const ctx = await mounted()
-    const child = session('sess-inherit-child')
+  it('overrideOf reads the header baseline when the log has no own switch', async () => {
+    const ctx = await mounted({ mode: 'workspace-write' })
+    const child = inheritedSession('sess-inherit-baseline', { sandboxMode: 'read-only' })
 
-    ctx.sandboxPolicy.stampOverride(child, 'read-only')
-
-    const stamped = modeEvents(child)
-    expect(stamped).toHaveLength(1)
-    expect(stamped[0]?.data).toEqual({ mode: 'read-only' })
+    expect(ctx.sandboxPolicy.overrideOf(child)).toBe('read-only')
+    // resolve() consumes the same chain, so enforcement sees the baseline.
+    expect(ctx.sandboxPolicy.resolve({ session: child }).mode).toBe('read-only')
   })
 
-  it('stampOverride skips a child already folding to the mode (fork-seed dedup)', async () => {
+  it('a seed-carried stale switch loses to the baseline; an OWN later switch wins over it', async () => {
+    const ctx = await mounted({ mode: 'workspace-write' })
+    // The fork seed carried the parent's OLD workspace-write switch (one
+    // event, so seedLength 1); the delegation-time baseline is read-only.
+    const child = inheritedSession('sess-inherit-slice', { sandboxMode: 'read-only', seedLength: 1 })
+    setSandboxMode(child, 'workspace-write')
+    expect(ctx.sandboxPolicy.overrideOf(child)).toBe('read-only')
+    // A switch the child makes ITSELF (after the seed boundary) outranks it.
+    setSandboxMode(child, 'danger-full-access')
+    expect(ctx.sandboxPolicy.overrideOf(child)).toBe('danger-full-access')
+  })
+
+  it('rejects a header baseline outside the closed mode vocabulary (durable boundary)', async () => {
     const ctx = await mounted()
-    const child = session('sess-inherit-dedup-child')
-    // A fork seed can already carry the parent's switch; stamping again would
-    // append a redundant event on every delegation.
-    setSandboxMode(child, 'read-only')
+    const child = inheritedSession('sess-inherit-invalid', { sandboxMode: 'yolo' })
 
-    ctx.sandboxPolicy.stampOverride(child, 'read-only')
-
-    expect(modeEvents(child)).toHaveLength(1)
+    expect(() => ctx.sandboxPolicy.overrideOf(child)).toThrow(/sandboxMode/)
   })
 })