feat: implement fail-closed unattended merge script (merge_pr.py)

[adamsithr]

Bounty Payout Address:

• Network: Solana (SOL)
• Address: 7jXAK7dDBwkAJT67WjPeu8cg577y3x1oMp7ZCSMV18mR

Summary

This PR implements the unattended fail-closed merge tool in merge_pr.py to satisfy Issue #13.

Acceptance Criteria Met

• Merge CLI (merge_pr.py): Implemented complete command-line interface validating:
  • Repository is in allowlist.
  • Trigger label hermes:auto exists and is applied by authorized collaborator (write/maintain/admin).
  • Branch protection and required status checks have successfully passed.
  • HEAD commit SHA matches the expected commit SHA exactly.
• Sensitive Path Guard: Configured globs (e.g. .github/workflows/*, pyproject.toml) blocking unattended merge if modifications are detected.
• Table-Driven Tests: Added comprehensive table-driven tests in tests/test_merge_pr.py covering success and failure scenarios for the policy gate matrix.
• Documentation: Updated README.md, docs/security-model.md, docs/enforcement-model.md, and SKILL.md to reflect Operating Mode 4 functionality.

Closes #13