Enhancement: Structured changelog management (per-chart + full repo)

## Summary

Implement structured changelog management that supports both per-chart changelogs and a full repository changelog.

## Current Behavior

The W1 workflow generates a changelog preview comment for changed charts using git-cliff with `--include-path`. This is useful for PR review but doesn't persist the changelog in a structured way.

## Desired Behavior

1. **Per-Chart Changelogs**: Each chart should have its own `CHANGELOG.md` maintained in `charts//CHANGELOG.md`
2. **Full Repository Changelog**: A top-level `CHANGELOG.md` aggregating all chart releases
3. **Integration with Release Process**: Changelogs should be updated as part of the release workflow (W4 or W5)

## Proposed Implementation Options

### Option A: Automated via release-please
- Configure release-please to maintain per-chart changelogs
- Use `extra-files` config to update chart-specific changelog files

### Option B: Custom git-cliff integration
- Use git-cliff with `--include-path` during release to update per-chart changelogs
- Use git-cliff without path filter for full repo changelog
- Add as a step in W4 or W5 workflow

### Option C: Hybrid approach
- release-please handles versioning and main changelog
- git-cliff handles per-chart changelogs in a pre-release step

## Considerations

- Should integrate with ArtifactHub changelog annotations (already in Chart.yaml)
- Must work with conventional commits structure
- Should not duplicate effort between PR preview and actual changelog

## Related

- Part of the 8-workflow release pipeline
- See `.claude/plans/chart-release-workflows/` for workflow context