Add helm values schema

As a programmer I would like a schema for `values.yaml` to know what values are available, and to validate a given values file for correctness.

### Acceptance Criteria
1. A helm values schema exists
2. The schema is referenced in the values.yaml for the yaml language server
3. The values file is tested against the schema as part of pre-commit / CI (e.g. with `helm template`)

Start with values.yaml and inspect the repository's existing pre-commit or CI configuration. Add a Helm values schema, reference it from values.yaml for YAML language-server validation, and make the values file validation run as part of pre-commit or CI, for example with helm template.

Written by the indexing model from the issue text.