agent-mesh: audit entry_hash and previous_hash always empty - tamper detection non-functional

## Problem
`AuditEntry` has `previous_hash` and `entry_hash` fields that are never computed. Audit log tamper detection is non-functional.

**File:** `src/agentmesh/governance/audit.py` (lines 55-56)

## Impact
Users relying on hash chaining for integrity get false security.

## Suggested Fix
Either implement hash chaining or mark fields as deprecated with runtime warnings.