feat(gate): separate CC-512 assurance coordinates

[screenleon]

Summary

• add canonical TSV policy sources for gate tiers, execution modes, and review pass kinds, with a parity-checked copy-mode fallback
• separate reviewer coverage from tier resolution, add canonical --mode, and require a valid --initial-result for targeted remediation passes
• update gate briefs, documentation, schema compatibility notes, and behavioral/parity tests

Why

Gate rigor, execution topology, review pass, and reviewer coverage were previously conflated. In particular, reviewer overrides could rewrite the tier to targeted, while targeted remediation could not reference its initial review.

This establishes the independent assurance coordinates required by CC-512 without coupling tier to mode or coverage.

Impact

• --reviewers changes requested coverage without changing the resolved tier
• --targeted is a remediation pass kind and requires --initial-result
• --mode sequential|parallel is canonical; compatible aliases remain supported and conflicting inputs fail closed
• legacy schema v1 artifacts may still contain the targeted tier, but current producers do not emit it
• this delivers CC-512 Slice A only; machine-owned envelope, evidence capture, and shared-verifier work remain in later slices

Gate

• Refactor/reuse audit: PASS — no warranted refactor/reuse change
• Implementation: Codex main thread
• Review executor/model: Claude / claude-sonnet-5
• Review rounds: 1 executed round
• Final verdict: GO; critic/QA/architecture/security/risk all passed
• Affected evidence: 15/15 suites passed, 0 failed, 0 skipped
• Gate operation: op-20260727T015241Z-569391

Full-suite validation

• tests/bin/run-all-tests.sh: 97 passed, 0 failed, 0 skipped
• tests/bin/run-tests.sh --verify-full .pm-dispatch/test-results/latest-full.json: PASS
• Verified commit: c73d29878fb2e309032009e1a8669b0421d48baa
• Verified tree fingerprint: 5f3fdefc990dc6db19c2a495c7d2f832c58c0e6fa7091449883f97627dce6803

Relates to CC-512 (Slice A).