docs(agents): document stale Helm subchart cleanup

[elezar]

Summary

Document how agents should handle stale Helm subchart artifacts when helm lint reports undeclared dependencies, using the postgresql cleanup path from PR #1947.

Related Issue

Related: #1947

Changes

• Added a Helm chart checks section to the Helm dev environment skill.
• Included targeted cleanup commands for stale deploy/helm/openshell/charts/postgresql artifacts.
• Clarified that charts/ is ignored and regenerated by helm dependency build for declared dependencies.

Testing

• mise run pre-commit passes - fails in this environment on unrelated openshell-supervisor-process current user/group lookup tests.
• Unit tests added/updated - not applicable; docs-only skill update.
• E2E tests added/updated (if applicable) - not applicable.
• mise run helm:lint passes.

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable) - not applicable; agent skill documentation only.

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[elezar]

/ok-to-test 9bf63e1