test(snapshot): pin the parent-only override through the assembled headless app

Review fix (ds-review-bot on #623): the ACP scenario runs at deployment
strength (the automation protocol has no session-scoped switch), so the
assembled-app path could not detect the delegation bypass itself. The new
keyless subagent-inheritance headless scenario closes that on the
semantic-checkpoint precedent: a seeded parent log carrying a real
sandbox/mode: read-only switch under a workspace-write deployment default
is resumed through the Loader-booted cli-demo app via a resume fixture
plugin and delegates through the real subagent tool; the child's real
write is denied by the real dsh-fs-sandbox fence (physical ENOENT
assertion), its persisted header carries the inherited baseline, and both
logs pin as expected outputs. Verified red: disabling the driver's capture
makes the scenario fail on the disk assertion (the child writes under the
deployment default).

examples/headless-agent/tests/fixtures/subagent-inheritance-agent.ts

@@ -0,0 +1,25 @@
+/**
+ * Loader fixture that resumes the seeded read-only parent before CLI dispatch.
+ * @module subagent-inheritance-agent
+ */
+
+import type { Context } from 'cordis'
+import type { SessionId } from '@deepseek-ai/dsh-session'
+
+/** Fixture plugin name. */
+export const name = 'subagent-inheritance-agent'
+/** Services that must exist before the fixture resumes its agent. */
+export const inject = ['agents', 'agentLoop', 'sessionPersistence']
+
+/**
+ * Resume the seeded session and bind its exact handle to this fixture's lifetime.
+ * @param ctx - settled agent and persistence services from the Loader tree.
+ * @returns after the resumed agent is published.
+ */
+export async function apply(ctx: Context): Promise<void> {
+  const handle = await ctx.agents.resume({
+    resumeSessionId: 'subagent-inheritance-parent' as SessionId,
+    agentOptions: { provider: 'deepseek', model: 'deepseek-v4-flash' },
+  })
+  ctx.effect(() => () => handle.dispose(), 'subagent-inheritance-agent.handle')
+}