Add helm chart linting to CI

[doxxx93]

Motivation

Closes #41

The project uses Helm charts but the CI lint job only covers Rust (rustfmt, clippy). There is no validation for the Helm chart structure or generated Kubernetes manifests.

Solution

• Add lint-helm recipe to justfile for local and CI use
• Integrate into the existing lint job in CI:
  • helm lint to validate chart structure and values
  • helm template | kubeconform --strict to validate generated manifests against Kubernetes schemas
• Tools installed in CI: azure/setup-helm@v4, kubeconform, extractions/setup-just@v3

[doxxx93]

@clux This addresses the same issue as #64 (draft). I took a slightly different approach using helm lint + helm template | kubeconform --strict with CLI installation instead of the Docker container method. Happy to adjust if you have any preferences!

[doxxx93]

Switched to the official kubeconform Docker image (docker://ghcr.io/yannh/kubeconform:latest) as described in the kubeconform README.

[clux]

looks great. thank you.