feat(run): add unified seat health probe

[solomonneas]

Closes #577

Implements the shared seat-health coordinator from #474 and the accepted proposal in docs/proposals/2026-07-26-seat-health-probe-and-worker-failure-taxonomy.md.

• probes requested seats and declared fallbacks in parallel
• covers declaration, executable, auth, transport, version, model, and isolation checks through an explicit adapter matrix
• enforces 30-second per-seat, 15-second model, and 35-second roster budgets
• caches by redacted seat fingerprint and always recomputes isolation compatibility
• writes atomic brigade.seat_health.v1 receipts with dual-clock timestamp handling
• adds the shared result to roster doctor without removing existing labels

Verification: ./scripts/verify passed through brigade work verify run, receipt 20260727-000622-work-verify-600d75 (4,506 passed, 3 skipped).

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