Stability checkpoint v0.5.3 — smoke:full-stack allow_failure (DinD bind-mount limitation)

- fix(ci): smoke:full-stack allow_failure + dated TODO (DinD bind-mount issue) (156d2c1)

v0.5.1 + v0.5.2 had smoke:full-stack fail on a Docker-in-Docker
bind-mount limitation : the macbook-local runner shares the host's
docker daemon, so paths under /builds/... in the CI workspace aren't
visible to the daemon. The dnsmasq.conf + tinyproxy.conf bind mounts
fail before any of the test logic runs.

Pragmatic short-term : `allow_failure: true` with a dated TODO
(2026-05-15) per CLAUDE.md global short-term-bridge exception. The
local `bin/test-full-stack.sh` still works (verified 2/2 PASS in v0.5.0
commit message + multiple local re-runs).

Tag pipeline goes from 31/32 (one hard failure) to 32/32 (1 allowed-
failure annotation) — green release object, signed images, cosign
verifies, registry tags all in place.

CI :
- ✅ Main pipeline green
- ✅ MR #20 merged
- ✅ Tag pipeline expected 32/32 (1 allow_failure)

- v0.5.4 — bake docker/dnsmasq.conf + docker/tinyproxy.conf into the
  sidecar images at build time so no bind mount is needed. Then drop
  the allow_failure here. Single-commit ticket.
- v0.6.0 — JWT validation + OIDC discovery + TTL cache + bearer auth
  on the IdP URL