feat: Task12B admin observability — agent-consumable diagnose UX [A2 2/5]

[law-chain-hot]

Summary

PR 2 of 5 (effort: A2). Stacks on #699. Adds Task12B admin observability — agent-consumable diagnose UX. Draft / WIP.

Incremental diff only (78 files); the foundation rename is in #699.

Before

Admin observability
+--------------------------------------+
| box logs / metrics / traces exist    |
| but no agent-consumable diagnose UX  |
| (a human digs through raw signals)   |
+--------------------------------------+

After

+--------------------------------------+
| admin diagnose UX, agent-consumable  |
| box-level logs + metrics surfaced    |
| in one place for triage              |
+--------------------------------------+

What Changed

• Agent-consumable admin diagnose surfaces (overview + per-box diagnose).
• saved-image fix; ESLint flat-config repair.
• Infra env wiring for observability is deferred to the final merge (kept out of this Draft to stay focused on the UX).

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