feat(helm): Chart.yaml dependencies -> DEPENDS_ON edges + values.yaml top-level keys

Two more Helm semantic improvements for #338:

- Chart.yaml: parse the top-level name and the dependencies: list
  (cbm_parse_helm_chart) and, in the k8s pass, emit a Chart node per chart plus
  a DEPENDS_ON edge to a shared, per-project deduplicated Chart node for each
  declared dependency. So "which charts depend on postgresql" is now traversable.
- values.yaml: extract only the top-level keys as structured Variables instead
  of one node per nested leaf key — killing the node flood the issue described
  (86 nodes for a typical values.yaml down to a handful).

Generic-YAML one-node-per-file dedup (repo-wide behavior change) is intentionally
left as a separate follow-up. Tests cover the dependency parser (with/without
deps) and the values.yaml top-level-only extraction.

Closes #338