feat(helm): enhance chart versioning for development and stable releases

[manusa]

Summary

Automates Helm chart releases via GitHub Actions, publishing to GHCR as OCI artifacts.

Continuation of #499

Changes

Makefile (build/helm.mk):

• Added targets: helm-lint, helm-validate, helm-package, helm-push, helm-verify
• Auto-installs kubeconform to _output/tools/bin/
• Dynamically sets appVersion from GIT_TAG_VERSION during packaging

Workflow (.github/workflows/release-helm.yaml):

• Tag push: Creates stable releases (e.g., 0.1.0)
• Manual trigger: Creates dev releases (e.g., 0.1.0-dev.cd5a70b3)
• Validates, packages, and pushes to ghcr.io/containers/charts/kubernetes-mcp-server

Chart:

• Added appVersion field
• Updated README with OCI registry installation instructions
• Fixed typos and template filename

Installation

helm install kubernetes-mcp-server \
  oci://ghcr.io/containers/charts/kubernetes-mcp-server \
  --set ingress.host=<hostname> --version 0.1.0 \
  --create-namespace --namespace mcp-system

[Cali0707]

@manusa can you update the README at some point to reflect these helm changes/versioning?