feat: harden gateway with policy engine, secure tools, and governance docs

docs/audit.md

@@ -0,0 +1,33 @@
+# Audit Logging
+
+## Design
+
+Audit logs are append-only JSON lines with hash chaining:
+- `prev_hash`: previous entry hash.
+- `entry_hash`: hash of current entry payload + previous hash.
+
+This makes tampering detectable.
+
+## Event Types
+
+- `tool_invocation`
+- `access_denied`
+- `security_event`
+
+Each event includes timestamps and correlation context.
+
+## Integrity Validation
+
+Use:
+
+```bash
+python3 scripts/validate_audit_log.py --path /var/log/aegis-mcp/audit.log
+```
+
+Exit code `0` indicates valid chain, non-zero indicates tamper/corruption.
+
+## Operational Expectations
+
+- Persist audit logs to durable storage.
+- Protect write permissions (service account only).
+- Validate integrity during incident response and release checks.