Migrate from 'charts' branch to 'release' branch

## Summary

Migrate the helm chart release infrastructure from the `charts` branch to the new `release` branch to align with the attestation-backed workflow architecture.

## Background

The `charts` branch currently serves as the source for:
- **charts.arusty.dev** - Helm repository index
- **ArtifactHub** - Package discovery

The new workflow architecture (W1-W8) uses:
- `integration/` branches for staging
- `main` branch for validated changes
- `release` branch for published releases

## Migration Tasks

- [ ] **Documentation Update**: Update all references from `charts` to `release`
- [ ] README.md (if applicable)
- [ ] ArtifactHub configuration (repository.yml or similar)
- [ ] charts.arusty.dev DNS/hosting configuration

- [ ] **GitHub Pages Configuration**
- [ ] Verify GitHub Pages source branch setting
- [ ] Update if currently pointing to `charts`

- [ ] **ArtifactHub Integration**
- [ ] Update ArtifactHub repository URL to point to `release` branch
- [ ] Verify charts are discoverable after change

- [ ] **Transition Strategy**
- [ ] Keep `charts` branch in sync during transition period
- [ ] Set deprecation notice on `charts` branch
- [ ] After verification, consider branch protection to prevent direct pushes

- [ ] **Validation**
- [ ] Verify helm repo add/update works with new branch
- [ ] Verify ArtifactHub shows charts correctly
- [ ] Test helm install from new repository URL

## Current State

- `release` branch created from `charts` (identical content)
- `charts` branch still active and serving traffic
- No references updated yet

## Notes

The `release` branch was created to support the W6 (Atomic Chart Tagging) workflow which creates tags and opens PRs to the release branch for final publication.

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*Related: W6 plan at `.claude/plans/chart-release-workflows/workflow-6/plan.md`*