- Add per-chartDir mutex via getNamedRWMutex to prevent concurrent
releases referencing the same local chart from racing on helm dep
build writes (same pattern as forcedDownloadChart for #768).
- Log non-lock errors at Warnf instead of Debugf so users can diagnose
why strategicMergePatches fail with "no resource matches".
- Document why --skip-refresh is kept (file://-only deps are the common
case; chartify runs dep build without --skip-refresh afterwards and
catches stale repos for mixed-dep subcharts).
- Use strings.HasPrefix instead of manual slicing in test helper.
Signed-off-by: yxxhero <aiopsclub@163.com>