Chart context: file scope and appVersion-derived tags

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## Parent

#17

## What to build

Two behaviours that both need to know which chart governs a values file. First, scope: values files anywhere beneath a chart directory are checked, subchart values included, and that chart's `templates` directory excluded. Second, references with no tag resolve through the governing chart's `appVersion`, matching Helm's own semantics.

The governing chart is the one in the **nearest ancestor directory containing chart metadata**, so a values file inside a subchart resolves against the subchart's own metadata rather than the parent's — which is what Helm would actually deploy. Messages about a tag taken from `appVersion` say so, and the diagnostic attaches to the repository value, since there is no tag in the file to underline; the reader needs to be pointed at the chart metadata rather than left hunting for a tag that is not there.

Absent chart metadata, or an absent `appVersion`, yields no marker.

Editing a chart's `appVersion` re-checks the open files that depend on it, because a version bump otherwise leaves stale checkmarks at exactly the moment correctness matters most.

This lives in the Helm package with filesystem reads injected. Passing `appVersion` in from the extension was considered and rejected in spec #17: which chart governs which values file is Helm knowledge, and leaving it in an editor extension guarantees a future CLI reimplements it.

## Acceptance criteria

- [ ] Values files beneath a chart directory are checked, not only files matching a values naming pattern
- [ ] Files under a chart's `templates` directory are never checked
- [ ] A subchart's values file resolves `appVersion` from the subchart's own metadata, not the parent chart's
- [ ] A tagless reference is checked against the resolved `appVersion`
- [ ] Its diagnostic attaches to the repository value and names chart metadata as the tag's source
- [ ] No chart metadata, or no `appVersion`, produces no marker
- [ ] Changing `appVersion` re-checks the affected open files
- [ ] Chart context resolution tested in the Helm package with filesystem reads injected, so tests describe a directory shape rather than writing temporary files

## Blocked by

- #20 — Structural detection of image references anywhere in a values file

Start in the Helm package and review spec #17 plus the filesystem-read injection points; this work is blocked by #20's structural image-reference detection. Define directory-shape tests for nested charts, templates exclusions, appVersion resolution, diagnostics, and re-checking after metadata changes; done means every acceptance criterion is covered without temporary files.

Written by the indexing model from the issue text.