Introduce yamllint to enforce consistent YAML style and detect errors across runner manifests (base, instance overlays, cluster overlays) in the k8s-gitlab-runner repository.

Example: https://gitlab.cee.redhat.com/releng/konflux-release-data/-/blob/main/.yamllint

Acceptance Criteria

• .yamllint file exists in repository root with agreed rules

• New GitLab CI job runs yamllint on every Merge Request

• Pipeline fails when lint errors occur

• Successfully linted existing repository content as part of MR

• README or docs updated with local usage instructions (yamllint -f auto .)

We should use the gitlabcee shared instance for this:

https://source.redhat.com/groups/public/gitlabcee/user_documentation/gitlabcee_shared_runner_faqs