feat: gsc-shell-api v0.1 — central chrome data API

Tiny Go service that returns ShellConfig JSON for any registered app.
Backs the runtime-loaded <AppShell> component being added to
@limitless/ui (next).

Endpoints:
  GET /api/v1/shell/{appKey}  → app + branding + user + menus, ETag-cached
  GET /api/v1/apps            → registered app inventory
  GET /healthz, /readyz       → ops probes

Auth:
  Keycloak Bearer JWT validated against the gosecCloud realm.
  Discovery URL is overridable so pods can hit Keycloak via the
  in-cluster service (https://keycloak.keycloak.svc.cluster.local:8443)
  while still validating the canonical issuer (auth.gosec.cloud).
  Lazy JWKS init — pod stays up if Keycloak is briefly unreachable.

Data model (gsc_core.shell):
  apps · menu_items (zone enum: topbar/sidebar/footer/user-menu) ·
  menu_role_grants (Keycloak realm roles, OR semantics, empty=all) ·
  branding

Seed includes the 8 gsc-crm sidebar items + topbar search +
user-menu (settings/support/logout) + footer (docs).

K8s:
  Namespace gsc-shell (ambient mesh).
  Deployment 2 replicas, internal-only ingress shell-api.gosec.internal,
  EJBCA SERVER cert.
  ServiceEntry for auth.gosec.cloud (vestigial — discovery now uses
  in-cluster path; keeping for ad-hoc curl from inside pods).
  Added to keycloak/allow-keycloak-clients AuthorizationPolicy out of band.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

k8s/namespace.yaml

@@ -0,0 +1,7 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: gsc-shell
+  labels:
+    app.kubernetes.io/name: gsc-shell
+    istio.io/dataplane-mode: ambient