refactor(policy): merge policy gateway into policy-engine

- Move 24 gateway source files (endpoints, services, contracts) into engine
  under Endpoints/Gateway/, Services/Gateway/, Contracts/Gateway/ namespaces
- Add gateway DI registrations and endpoint mappings to engine Program.cs
- Add missing project references (StellaOps.Policy.Scoring, DeltaVerdict, Localization)
- Remove HTTP proxy layer (PolicyEngineClient, DPoP, forwarding context not copied)
- Update gateway routes in router appsettings to point to policy-engine
- Comment out policy service in docker-compose, add backwards-compat network alias
- Update services-matrix (gateway build line commented out)
- Update all codebase references: AdvisoryAI, JobEngine, CLI, router tests, helm
- Update docs: OFFLINE_KIT, configuration-migration, gateway guide, port-registry
- Deprecate etc/policy-gateway.yaml.sample with notice
- Eliminates 1 container, 9 HTTP round-trips, DPoP token flow

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/operations/configuration-migration.md

@@ -163,10 +163,13 @@ authority:
     - ../../etc/certificates/signing:/app/etc/signing:ro
 ```
 
-### Policy Gateway
+### Policy Engine (formerly Policy Gateway)
+
+> **Note:** The `policy-gateway` service has been merged into `policy-engine`.
+> Mount policy configuration on the `policy-engine` service instead.
 
 ```yaml
-policy-gateway:
+policy-engine:
   volumes:
     - ../../etc/policy:/app/etc/policy:ro
 ```