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

docs/delegated-mode-operations.md

@@ -6,7 +6,7 @@ Status: implemented for FLEX-WP-0004 P4.6.
 
 Delegated mode lets flex-auth coordinate external authorization systems
 without changing the protected-system-facing API. Protected systems keep
-using flex-auth check, batch, list, explanation, and audit contracts.
+using access-engine check, batch, list, explanation, and audit contracts.
 Topaz, relationship PDPs, rule PDPs, Keycloak Authorization Services,
 and directory resolvers stay behind adapter boundaries.