feat(agent): autoMaintain config block (.gittensory.yml + DB + dashboard)

[JSONbored]

Parent epic: #768 (Phase 0).

Goal

A new autoMaintain section in config-as-code, resolved with the existing yml>DB>defaults precedence, surfaced in the dashboard.

Deliverables

• autoMaintain.* schema (per-action autonomy, merge/close policy, requireApprovals)
• Loader + precedence (reuse focus-manifest-loader)
• Dashboard read/write (ties to F1/N12)

Context

src/signals/focus-manifest.ts, focus-manifest-loader.ts, .gittensory.yml.

Milestone: M2.