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]>

conformance/incluster/kustomization.yaml

@@ -0,0 +1,14 @@
+# Reconciled by clusters/base/conformance.yaml (non-gating, wait:false).
+#
+# RBAC ONLY. chainsaw-job.yaml is deliberately NOT listed: that manifest is a TEMPLATE the CI
+# trigger renders and submits per run (with the commit SHA and the touched --requirements). Flux
+# reconciling it would create a permanent Job object and fight the trigger for ownership.
+#
+# The identities here are inert without
+# platform/kyverno/admission/conformance-trigger-job-shape.yaml, which is what actually bounds
+# the trigger to the committed run shape. RBAC is the necessary half, that policy the sufficient
+# half — see the header of rbac.yaml.
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+resources:
+  - rbac.yaml

conformance/incluster/rbac.yaml

@@ -174,3 +174,68 @@ subjects:
   - kind: ServiceAccount
     name: conformance-runner
     namespace: conformance
+---
+# =============================================================================
+# THE TRIGGER IDENTITY — deliberately NOT conformance-runner.
+#
+# THE ESCALATION THIS EXISTS TO CLOSE (docs/plans/l2-l3-conformance-ci-wiring.md §4):
+# Kubernetes places NO `use` check on a Pod/Job's `serviceAccountName`. So any principal
+# that can `create jobs` in this namespace can create a Job with
+# `serviceAccountName: conformance-runner` and an ARBITRARY command — i.e. run arbitrary
+# code with the runner's privileges (nodes/patch, pods/exec, namespaces/patch, jobs).
+# A CI-held token with plain `create jobs` therefore IS the privileged identity, no matter
+# how narrow its own RBAC looks. That would break R-SEC-02 ("CI holds no broad cluster
+# access") while appearing to satisfy it.
+#
+# RBAC ALONE CANNOT FIX THIS. The real boundary is the admission policy
+# platform/kyverno/admission/conformance-trigger-job-shape.yaml, which admits Jobs from
+# THIS ServiceAccount only when they match the committed conformance shape. This Role is
+# the necessary half; that policy is the sufficient half. Neither works alone — do not
+# grant this Role without that policy reconciled and ENFORCING.
+#
+# Namespaced Role, bound only in `conformance`. No ClusterRole, deliberately.
+# =============================================================================
+apiVersion: v1
+kind: ServiceAccount
+metadata:
+  name: conformance-trigger
+  namespace: conformance
+  labels:
+    app.kubernetes.io/part-of: conformance
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: Role
+metadata:
+  name: conformance-trigger
+  namespace: conformance
+  labels:
+    app.kubernetes.io/part-of: conformance
+rules:
+  # Create the run, and watch it to completion.
+  - apiGroups: [batch]
+    resources: [jobs]
+    verbs: [create, get, list, watch]
+  # Read the Job's pod and its log — this is how results come back. Deliberately NOT
+  # pods/exec and NOT `kubectl cp`: the run tars its artifacts to stdout and CI decodes
+  # them from the log, so no execution channel into the runner pod is ever granted.
+  - apiGroups: [""]
+    resources: [pods, pods/log]
+    verbs: [get, list, watch]
+  # NOTHING ELSE. In particular NO delete on jobs (a trigger that can delete can hide a
+  # failed run), no secrets, no configmaps, no exec, no create on pods.
+---
+apiVersion: rbac.authorization.k8s.io/v1
+kind: RoleBinding
+metadata:
+  name: conformance-trigger
+  namespace: conformance
+  labels:
+    app.kubernetes.io/part-of: conformance
+roleRef:
+  apiGroup: rbac.authorization.k8s.io
+  kind: Role
+  name: conformance-trigger
+subjects:
+  - kind: ServiceAccount
+    name: conformance-trigger
+    namespace: conformance