ci: run verify-stack only on push to main, not on PRs (refs #182)

The runner shares the lab node with the deployed stack; a second full stack
per PR is what got it OOM-killed. PRs keep every other job as the merge gate.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/runbooks/ci.md

@@ -21,7 +21,7 @@ and CI cannot drift:
 | `frontend` | `make frontend` → Nx lint/test/build for the four portals | pnpm + Node |
 | `k8s` | `make k8s-lint` (render + schema-check the Helm chart) → `make k8s-drift` (chart still describes the same stack as `infra/docker-compose.yml`) | pinned `helm` binary + `docker compose` |
 | `mutation` | `make mutation` → `dotnet tool restore` → `dotnet stryker` (ACL); uploads the HTML report as an artifact | .NET 10 SDK |
-| `verify-stack` | the single live-stack stage — steps: `make verify-up` (full stack up + health, the DoD smoke) → `make verify-acl` (ACL ↔ OpenZaak) → `make verify-nrc` (OpenZaak → NRC delivery) → `make down` | container engine + egress (base images, nuget, `selectielijst.openzaak.nl`) |
+| `verify-stack` | **push to `main` only, skipped on PRs** (#182) — the single live-stack stage — steps: `make verify-up` (full stack up + health, the DoD smoke) → `make verify-acl` (ACL ↔ OpenZaak) → `make verify-nrc` (OpenZaak → NRC delivery) → `make down` | container engine + egress (base images, nuget, `selectielijst.openzaak.nl`) |
 
 > **Why one `verify-stack` job, not three.** The single self-hosted runner runs jobs
 > **sequentially**, so booting OpenZaak once (instead of once per check) is the