Tasks: spawned sessions never receive a secret

Task list

[x] A1: the boundary. envstate.credential_names, passthrough_names, SessionEnvironment, session_environment, spawn_environment, compare; the schema key. Req: R1.3, R4 · Test: T1, T9.
[x] A2: the runner. -e from forward only; env -u prefix; scrubbed tmux environment. Req: R1, R2 · Test: T2, T3, T4. Deps: A1.
[x] A3: drift and restart. _declared_environment, environment_drift, restart_sessions on the new boundary. Req: R5 · Test: T8. Deps: A1, A2.
[x] B1: routing. daemon_spawned, the no-fallback branch of harness_routed. Req: R3.2–R3.4 · Test: T5.
[x] B2: the ask route. Contract first, then route, facade, actor argument and the CLI. Req: R3.1 · Test: T6, T7. Deps: B1.
[x] C1: docs. Config reference, sessions and status command docs, the issue-410 security note, capability docs, the changelog's upgrade note. Req: R6 · Deps: A3, B2.
[x] C2: verify and ship. Run the testing plan, record the evidence, self-review, security review, reviewer briefing, PR. Test: T10, T11. Deps: C1.