fix: auto-close spawned children on completion; subagent shutdown ordering

[Pizzaface]

Problem

Spawned child sessions had two lifecycle bugs:

1. Completed spawn_session children never exited. PIZZAPI_WORKER_AUTO_CLOSE was only plumbed through the trigger-listener routes — the /api/runners/spawn path used by spawn_session never set it, so every spawned child idled on the runner forever after finishing its task. Killing the parent didn't help: children are independent worker processes by design.
2. Shutdown ordering orphaned subagent mirrors. subagentExtension registered after remoteExtension, so on session_shutdown the remote extension disconnected the worker socket before the subagent extension could abort in-flight relay mirrors and end those ephemeral child sessions — leaving linked children showing as alive until the 10-minute ephemeral TTL sweep.

Closes the shutdown-ordering half of Godmother idea jj0WhkKm.

Changes

• spawn_session tool: new autoClose param, default true. Plumbed through POST /api/runners/spawn → new_session → spawner env (PIZZAPI_WORKER_AUTO_CLOSE). Completed children now self-terminate instead of idling on the runner. The existing auto-close guard still applies — it won't close if new messages, subscriptions, or linked children arrive, and autoClose: false opts out for interactive follow-ups.
• factories.ts: register subagentExtension before remoteExtension so shutdown aborts mirrors and ends their relay sessions before the worker socket disconnects.
• Docs: autoClose documented in features/multi-agent.mdx.

Not in scope

Silent parent-link drop when the parent's Redis record is missing at child registration (child then renders top-level in the UI) — captured as Godmother idea ZwI6GkdP.

Testing

• bun run typecheck clean.
• 38 tests across touched files pass, incl. 2 new spawn_session autoClose plumbing tests (default true, explicit false).
• Full extensions/ + routes/ batch has 7 pre-existing cross-file pollution failures reproduced identically on clean origin/main.