feat: add sync validation to detect relayer/chain sync drift

[dominiccreates]

this pr closes #24 Summary
Introduces a state-consistency check that validates a state_hash included in PR data against a deterministic SHA-256 hash of the PR contents. This helps surface sync drift between the relayer and the chain.

Changes

• src/audit-guard/src/sync-validator.ts – Implements SyncValidator with hash computation.
• src/audit-guard/src/errors.ts – Defines SyncDriftError.
• src/audit-guard/src/policy-engine.ts – Imports the validator, runs it during evaluation, and adds a SYNC_DRIFT_DETECTED violation (severity HIGH) when drift is detected.
• Updated PRData interface to include optional state_hash.

Verification

• Checked unit test configuration and local execution.

[N-thnI]

please fix checks