docs(run): propose seat health failure taxonomy

[solomonneas]

Tracks #474.

Summary

• inventory current roster, doctor, transport, dispatch, and receipt behavior
• define a closed worker-failure taxonomy and adapter-owned seat health probe
• specify additive receipt shapes, routing rules, migration, and compatibility
• separate the implementation into five ordered S/M tracker items

This is a planning-only change. It does not implement the probe or alter run behavior.

Verification

• brigade work verify run --target . --command "./scripts/verify" --capture brigade-work
• completed: 20260726-223358-work-verify-e5e891

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[solomonneas]

Blocking merge on a receipt-contract mismatch with merged #562:

• The brigade.seat_health.v1 example omits integer schema_version.
• The nested brigade.worker_failure.v1 example also omits integer schema_version.
• The proposal says seat-health.json is updated in place after re-probes, but does not classify it as a latest-state snapshot or preserve probe events. That conflicts with #562s write-once/append-only policy unless the mutable snapshot semantics are explicit.
• The new writers do not specify canonical sorted-key serialization, absent-versus-null rules, or reader compatibility.

Please rebase onto main and define those contract points before implementation is split from this proposal. An implementer following the current examples would produce receipts that fail the policy merged in #562.