docs(supervise): fix the example's deliverable check + self-diagnosing output

[drewstone]

Fixes three real DX bugs in the flagship supervise example, found by running it live:

1. Broken deliverable check — String(out).includes('READY') where a router-tools worker settles { content: string }; String({content}) is '[object Object]', so a correct delivery never matched and the run always read as no-winner. Now reads out.content.
2. Opaque failure — printed a bare no winner (kind); now reports the typed reason, child-down count, and spend on a no-winner (and the delivered output on a win), so a failure explains itself.
3. Prompt + comment — an explicit numbered spawn→await_event→stop protocol (with "never stop while a worker is still running"), and a comment explaining why the demo overrides defaultSupervisorPrompt (a tool-less supervisor whose only path to a credited deliverable is a spawned child).

Note for reviewers: a live run can still show no-winner when the router brain (esp. weaker models) spawns-and-stops without awaiting its worker — that's brain pull-discipline, not a runtime bug (the runtime correctly abandons an in-flight worker rather than deadlock-waiting). The self-diagnosing output now makes that visible. Typecheck + lint green.

[tangletools]

✅ Auto-approved drewstone PR — 53a8554d

This PR was opened by the trusted drewstone account.
The full PR reviewer audit still runs separately and will publish findings if it detects issues.

_{tangletools · auto-approval · reason: drewstone_author · 2026-07-02T10:44:12Z}