feat: add bootstrap integrity verification for governance modules

[imran-siddique]

Problem

The governance toolkit audits agent behavior with cryptographic chains, but does not verify its own integrity. A supply chain attack on any of the five packages could silently neuter enforcement while logging clean results. ''Who watches the watcher'' is unaddressed.

What Exists

• ✅ Merkle chain audit trail with hash verification
• ✅ Plugin signature verification (Ed25519)
• ✅ SBOM + GitHub attestations on release
• ❌ No module hash verification at startup
• ❌ No runtime watchdog for policy engine code
• ❌ No code signing on PyPI publish (trusted publishing only)

Proposed Solution

1. Bootstrap integrity: Hash all governance module files at startup, compare against published manifest
2. Runtime watchdog: Periodic __code__ hash check on critical enforcement functions
3. Package signing: Sign wheel files with Sigstore during release
4. Integrity manifest: Ship integrity.json with SHA-256 hashes of all source files

/cc @imran-siddique