chore: defer engine-specific policy-matrix workflow to GA

Removes .github/workflows/policy-matrix.yml. Engine-specific admission
validation (PSA-restricted × Kyverno × Gatekeeper × default+production)
delivered negative ROI at rc1: 4 PRs blocked on flaky CRD-bootstrap +
upstream-version races (#475, #481, #498, #501). Caught zero real
regressions; only its own infra bugs.

Coverage retained via conftest (offline policy), helm lint, kubeconform,
and kubectl apply --dry-run=server in chart.yml. policy-matrix-smoke.sh
+ bundles stay in tree for cheap reactivation at GA.

Triggers for re-enabling:
  - GA criterion #1 (third-party audit) requests engine-specific compat
  - First operator running under Kyverno/Gatekeeper reports admission rot
  - CRD-bootstrap pattern stabilised across other workflows

Tracking: #502 (filed alongside).
Signed-off-by: Tri Lam <tree@lumalabs.ai>

Author: trilamsr

.github/actions/kind-cluster-setup/action.yml

@@ -1,7 +1,7 @@
 name: Set up kind cluster (with optional CRD prereqs + tracecore image)
 description: >
-  Unified kind-cluster bootstrap shared by chart.yml, policy-matrix.yml,
-  install-bench.yml, and (optionally) compat-matrix.yml. Replaces the
+  Unified kind-cluster bootstrap shared by chart.yml, install-bench.yml,
+  and (optionally) compat-matrix.yml. Replaces the
   prior `kind-tracecore-up` action (deleted — all callsites migrated)
   and centralises three sources of fragmentation that recurred on every
   chart-touching PR:
@@ -11,7 +11,7 @@ description: >
     2. Each workflow that talked to the chart's production preset hit
        the same "no matches for kind ServiceMonitor in version
        monitoring.coreos.com/v1" error on kind (regressed three PRs
-       before #494 closed it for policy-matrix only). Install the CRDs
+       before #494 closed it). Install the CRDs
        in one place driven by inputs (`install-servicemonitor-crd`,
        `install-gatekeeper-crds`, `install-cert-manager-crds`).
     3. `docker build` + `kind load` for the tracecore image was