feat(sec): constrain the conformance CI trigger at admission (R-SEC-02)

Implements the credential model the operator chose for wiring L2/L3 into CI: a narrow
trigger SA bounded by a fail-closed Kyverno admission policy, rather than the
zero-credential GitOps-native alternative (plan section 4, primary model).

THE ESCALATION THIS CLOSES. Kubernetes places NO `use` check on a Job's
serviceAccountName. Any principal that can `create jobs` in ns conformance can create a Job
running as conformance-runner -- nodes/patch, pods/exec, namespaces/patch -- with an
arbitrary command. A CI token with plain `create jobs` IS that privileged identity no matter
how narrow its own Role looks. RBAC cannot fix this; only constraining the SHAPE of what the
trigger may create can.

Adds:
  * SA/Role/RoleBinding conformance-trigger -- create+get+list+watch jobs, get pods and
    pods/log, nothing else. Namespaced. No delete (a trigger that can delete can hide a
    failed run), no exec, no secrets. Results come back as a tarball on stdout read via
    pods/log, so no execution channel into the runner pod is ever granted.
  * ClusterPolicy conformance-trigger-job-shape -- Enforce, failurePolicy: Fail. It does NOT
    use ${KYVERNO_VALIDATION_ACTION}: a security boundary an overlay can downgrade to Audit
    is not a boundary. It constrains ONLY conformance-trigger, so a mistake here cannot wedge
    the rest of the cluster.
  * Flux Kustomization `conformance` -> ./conformance/incluster, wait:false and non-gating
    like apps-vms, so a test identity can never stall the platform or app tiers.
  * A kyverno test with seven cases, six of them real escalation paths.

VERIFIED by `kyverno apply` (pass: 10, fail: 6, error: 0), not assumed:
  evil-command    DENIED at /spec/template/spec/containers/0/command/   <- THE attack
  evil-image      DENIED at .../image/
  tag-not-digest  DENIED at .../image/          (mutable tag is republishable)
  host-namespaces DENIED at /spec/template/spec/hostPID/
  extra-volume    DENIED by the volumes foreach
  injected-args   DENIED by the args foreach
  good-run        ADMITTED

ALSO FIXES A FAIL-OPEN IN THE GATE ITSELF. `kyverno test` does NOT fail when a policy cannot
LOAD: every row becomes "Skip / Invalid Policy" and it still prints "N tests passed" and
exits 0. Measured while writing this policy -- a schema error (subjects in a separate
match.all entry from resources) produced exactly that, and ci/l0.sh would have certified a
security policy that does not load as green. This is not specific to this policy: a typo in
restrict-host-namespaces or secret-provenance would silently disable it with the gate still
green. ci/l0.sh now scans for "Invalid Policy" and fails. Proven: broken policy -> l0 rc=1,
restored -> rc=0.

IMAGE PINNING, HONEST LIMIT: the plan specifies pinning the exact runner image digest. That
image does not exist yet, so this enforces the enforceable half -- the image must be under
the conformance repository path AND digest-pinned, never a mutable tag. Residual surface:
anyone who can PUSH to that path could publish a different digest. Tighten to the literal
digest when the image is built; it is a one-line change to the pattern, reviewed in a PR,
which is where a change to a security boundary belongs.

NOT YET WIRED, deliberately: no runner image, no trigger kubeconfig, no CLUSTER_AVAILABLE.
The identities here are inert until those exist, and PR #12 makes CLUSTER_AVAILABLE=true
block rather than fake a green in the meantime.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

ci/l0.sh

@@ -79,12 +79,37 @@ else
   cp -a "${KYVERNO_POLICY_DIR}/." "$l0_render/"
   find "$l0_render" -type f -name '*.yaml' -print0 \
     | xargs -0 sed -i "s/\${KYVERNO_VALIDATION_ACTION}/${KYVERNO_VALIDATION_ACTION}/g"
-  if kyverno test "$l0_render"; then
-    ok "Kyverno L0 policy tests passed"
-    summary "- kyverno-l0: **passed**"
+  # DO NOT TRUST `kyverno test`'s EXIT CODE ALONE. A policy that fails to LOAD (schema
+  # error, bad match block, malformed JMESPath) does not fail the run: every result row
+  # becomes `Skip / Invalid Policy` and the CLI still prints "N tests passed" and exits 0.
+  #
+  # MEASURED 2026-09-29 while authoring conformance-trigger-job-shape: the policy had its
+  # `subjects` in a separate `match.all` entry from its `resources` — a schema error. All 9
+  # rows came back `Skip / Invalid Policy`, and the summary read:
+  #     Test Summary: 9 tests passed and 0 tests failed
+  # `kyverno apply` on the same file reported `error: 1` and named the path. So the gate's
+  # own policy stage would have certified a SECURITY POLICY THAT DOES NOT LOAD as green.
+  #
+  # This is not specific to that policy: a typo in restrict-host-namespaces or
+  # secret-provenance would silently disable it and the gate would stay green. Scan the
+  # output for the load failure the exit code hides.
+  l0_kout="$(mktemp)"
+  if kyverno test "$l0_render" 2>&1 | tee "$l0_kout"; then
+    if grep -q 'Invalid Policy' "$l0_kout"; then
+      err "Kyverno reported 'Invalid Policy' — one or more policies FAILED TO LOAD and were"
+      err "  skipped, so their rules were never evaluated. The CLI exits 0 for this; the gate"
+      err "  must not. Run 'kyverno apply <policy> --resource <resources>' for the exact path."
+      grep -n 'Invalid Policy' "$l0_kout" | head -5 >&2
+      summary "- kyverno-l0: **FAILED — policy did not load** (Invalid Policy; rules never evaluated)"
+      rc=1
+    else
+      ok "Kyverno L0 policy tests passed"
+      summary "- kyverno-l0: **passed**"
+    fi
   else
     err "Kyverno L0 policy tests failed"; summary "- kyverno-l0: **FAILED**"; rc=1
   fi
+  rm -f "$l0_kout"
   rm -rf "$l0_render"
 fi