feat: harden squash merge provenance

[andrei-hasna]

Description

• Add the repository-canonical agent-skills/merge-pr workflow with exact-head, reviewer, risk, repair-cycle, preflight, and merge-mode gates.
• Build deterministic squash argv with explicit subject/body and full preflight plus command digest correlation.
• Verify the actual provider merge commit and persist authoritative or non-authoritative receipts without exposing message text.
• Add raw regression fixtures for the multi-commit synthesis incident and focused repository boundary coverage.

The historical hasna/tai#4 incident is evidence only; this PR does not rewrite, revert, force-push, or otherwise change protected history.

Type of change

• Bug fix
• New feature
• Breaking change
• Refactor
• Documentation
• Chore (dependency updates, CI, etc.)

Validation

• Guard behavior suite: 24/24
• Agent workflow integration: 4/4
• Full isolated suite: 731/731
• bun run typecheck
• bun run build
• Release guard and package dry-run
• Skill validation and public-corpus drift check
• Two independent exact-head reviews with no P0-P3
• Fresh archived forward matrix: bundled 24/24 and independent 24/24
• Redacted Gitleaks scan and commit-trailer scan

Recovery and rollback

Failed provider verification preserves a durable failed receipt and never triggers automatic protected-history mutation. If the workflow itself must be rolled back, land an ordinary reviewed revert PR, then repeat exact-head review and release gates before further merges.

Checklist

• Tests pass (bun test)
• Typecheck passes (bun run typecheck)
• Documentation updated