docs: propose ProvenanceEnvelope contract

[solomonneas]

Summary

• defines brigade.provenance-envelope.v1 for evidence items and inter-seat messages
• maps eight ingestion boundaries and eleven consumption boundaries across MiseLedger and Python surfaces
• specifies trust authority, fail-closed injection handling, inferred legacy backfill, tamper response, and bounded hashing cost
• reconciles the contract with evidence ingestion: origin-scoped redaction policy with counted, non-stored redactions #498, evidence briefs: explain why each item was selected (rule, trust label, omitted count) #495, Causal lineage links across plan/run/verify/outcome/handoff (ProvenanceEnvelope companion record) #493, context packs: freshness state and lineage (source snapshot, generator hash, dependent receipts) #492, tool catalog: CandidateSetGate - least-privilege tool sets per task, recorded in receipts #504, and merged proposal work for run: unify seat preflight into a general health probe with a typed worker-failure taxonomy #474 and outcome ledger: multi-dimension skill scorecards from verifier receipts only, with exploration caps #502
• decomposes implementation into six ordered S/M issues

This is a planning-only change. It tracks #505 and does not close it.

Verification

brigade work verify run --target . --command "./scripts/verify" --capture brigade-work

• receipt: 20260726-231218-work-verify-670e1a
• result: 4,412 passed, 3 skipped
• coverage: 82.86%

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