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

examples/secrets-engine/replay/README.md

@@ -15,7 +15,7 @@ to verify its digest join (`627810b`) unchanged. `FLEX-WP-0021-T03`.
 Regenerate either with:
 
 ```bash
-go run ./cmd/flex-auth check \
+go run ./cmd/access-engine check \
   -policy   examples/secrets-engine/policy_package.md \
   -registry examples/secrets-engine/registry_snapshot.json \
   -request  examples/secrets-engine/check_request_allow_rotate.json