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>

devops/compose/hosts.stellaops.local

@@ -21,7 +21,7 @@
 127.1.0.12 vexlens.stella-ops.local
 127.1.0.13 vulnexplorer.stella-ops.local
 127.1.0.14 policy-engine.stella-ops.local
-127.1.0.15 policy-gateway.stella-ops.local
+127.1.0.14 policy-gateway.stella-ops.local  # backwards-compat alias (merged into policy-engine)
 127.1.0.16 riskengine.stella-ops.local
 127.1.0.17 orchestrator.stella-ops.local
 127.1.0.18 taskrunner.stella-ops.local