Push notifications when a specialist is waiting on you

Follow-up from the design in `docs/superpowers/specs/2026-08-31-bench-over-firestore-design.md`, deliberately out of scope for #45, #46 and #47.

## Why

Working from a phone is two halves. #46 gives the first: you can open the cockpit anywhere and drive a specialist. This is the second: being told that a specialist is waiting on you, without having the app open.

Without it, remote bench is something you have to remember to check. A specialist that hits a decision at 11am and is answered at 4pm has been idle for five hours.

## What it needs

- FCM, and a per-device subscription written when a device turns notifications on.
- A push handler in the service worker (`src/client/sw.ts` exists) that opens the cockpit at the waiting decision.
- A trigger in the daemon: it already knows the moment a turn ends with a decision — that is what the roster socket reports (`src/daemon/server.ts:1073`).
- A rule about what is worth waking someone for. A decision, yes. A reply, probably not.

Worth confirming before building: whether FCM's send path is usable without Cloud Functions, given this project is on the Spark plan and has no billing.

## Related

- Depends on #45 (identity) and #46 (the wire).
- Design: `docs/superpowers/specs/2026-08-31-bench-over-firestore-design.md`

Read docs/superpowers/specs/2026-08-31-bench-over-firestore-design.md, then inspect src/client/sw.ts and src/daemon/server.ts:1073. First confirm whether FCM sending works without Cloud Functions on the Spark plan, and resolve which events should wake a user. Done means device subscriptions, decision-triggered pushes, and cockpit links opening at the waiting decision work end to end.

Written by the indexing model from the issue text.