chore: Tag gha actions as v1 for stable references

## Context

The composite actions in this repository are currently referenced using `@main` in the helm-charts workflows. For production stability, these should be tagged as `v1`.

### Actions to Tag

All actions under:
- `actions/attestation/*`
- `actions/atomic-branch/*`
- `actions/changelog/*`
- `actions/dispatch/*`
- `actions/source-pr/*`
- `actions/trust-check/*`
- `actions/version-bump/*`

### Proposed Versioning Strategy

1. Create `v1.0.0` tag at current `main` HEAD
2. Create `v1` tag pointing to `v1.0.0`
3. Update helm-charts to use `@v1` instead of `@main`
4. Document versioning policy in README

### Benefits

- Stable references for production workflows
- Ability to make breaking changes on `main` without affecting consumers
- Clear semantic versioning for action consumers

### Acceptance Criteria

- [ ] All actions tagged as `v1.0.0`
- [ ] Floating `v1` tag created
- [ ] helm-charts workflows updated to use `@v1`
- [ ] Versioning policy documented

### Related

- helm-charts workflow migration complete
- Actions currently used with `@main` reference