Run golangci-lint in runner CI (not just go vet)

The repo ships a `runner/.golangci.yml` config and the `make lint` target supports golangci-lint, but the runner CI workflow should actually run `golangci-lint` rather than only `go vet`. This catches a much wider class of issues on every PR.

### Goal
Ensure `golangci-lint` runs in CI for the runner using the committed `.golangci.yml`.

### Scope
- Verify whether `.github/workflows/runner-lint-test.yaml` currently runs `golangci-lint`. If it only runs `go vet`, add the official `golangci/golangci-lint-action` step (pinned version) pointed at the `runner/` module.
- Make sure it uses the existing `runner/.golangci.yml` config.
- Confirm the job passes on the current `main` (fix or report any newly surfaced lint findings; large fixes can be split into follow-up PRs).

### Pointers
- `.github/workflows/runner-lint-test.yaml`
- `runner/.golangci.yml`
- `runner/Makefile` (`make lint`)

### Acceptance
- CI runs `golangci-lint` on PRs that touch the runner and the check is green on `main`.