feat: add cmux module (#523)

## Description

<!-- Briefly describe what this PR does and why -->

## Type of Change

- [x] New module
- [ ] New template
- [ ] Bug fix
- [ ] Feature/enhancement
- [ ] Documentation
- [ ] Other

## Module Information

<!-- Delete this section if not applicable -->

**Path:** `registry/[namespace]/modules/[module-name]`  
**New version:** `v1.0.0`  
**Breaking change:** [ ] Yes [ ] No

## Template Information

<!-- Delete this section if not applicable -->

**Path:** `registry/[namespace]/templates/[template-name]`

## Testing & Validation

- [x] Tests pass (`bun test`)
- [x] Code formatted (`bun fmt`)
- [x] Changes tested locally

## Related Issues

<!-- Link related issues or write "None" if not applicable -->

Author: ibetitsmike