[dmt] check helmignore root coverage

[diyliv]

Summary

• New check: .helmignore must cover every file and directory in the module root, except the standard Helm chart template entries (templates/, charts/, Chart.yaml, values.yaml).
• Uses Helm ignore engine: leverages helm.sh/helm/v3/pkg/ignore for proper pattern matching (wildcards, negation, directory-only rules).
• Affected files: pkg/linters/module/rules/helmignore.go, pkg/linters/module/rules/helmignore_test.go, e2e test fixtures.

Context

When a new directory (e.g. images/, hooks/) or file (e.g. go.mod, module.yaml) is added to a module root, developers often forget to add it to .helmignore. These files then leak into the Helm chart, increasing its size.

The rule now scans the entire module root and flags any entry that is not covered by .helmignore, except the four standard Helm template entries that must remain in the chart.

Example

Before: module has images/ and go.mod but .helmignore only lists hooks/ — no error.

After:

Error: Directory 'images/' is not listed in .helmignore
Error: File 'go.mod' is not listed in .helmignore

Excluding resources

linters-settings:
  module:
    helmignore:
      disable: true   # disables the entire rule