FLEX-WP-0036: binary renamed to access-engine; flex-auth is a stub naming the rename (FLEX-DEC-2026-020)

- cmd/flex-auth -> cmd/access-engine; usage and serving lines say access-engine
- cmd/flex-auth: prints the rename and exits 2, with a test
- image: entrypoint /usr/local/bin/access-engine, flex-auth stub alongside;
  live pins pass only args, so their next roll is unaffected
- Makefile, CI, verify-posture, tenancy.yaml, live docs updated; workplans,
  decisions and ADRs left as written
- runtime names (module, audience, k8s, image package, /opt paths) unchanged

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 735967@bnt-lap001
Assistant-Session: 5971967f-77f8-4ced-9e02-c6ef67040ffc

tenancy.yaml

@@ -124,7 +124,7 @@ evidence:
   A2:
     - "internal/callerauth/auth.go"
     - "internal/callerauth/auth_test.go"
-    - "cmd/flex-auth/main_test.go"
+    - "cmd/access-engine/main_test.go"
     - "charts/flex-auth/templates/rbac.yaml"
     - "deploy/caller-auth-rbac.yaml"
     - "docs/adr/0004-inbound-caller-authentication.md"