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>

etc/policy-gateway.yaml.sample

@@ -1,3 +1,8 @@
+# DEPRECATED: The Policy Gateway has been merged into Policy Engine.
+# This file is kept for reference only. Use policy-engine.yaml instead.
+# See docs/modules/policy/guides/gateway.md for migration guidance.
+#
+# Original description:
 # StellaOps Policy Gateway configuration template.
 # Copy to ../etc/policy-gateway.yaml (relative to the gateway content root)
 # and adjust values to fit your environment. Environment variables prefixed with