Add support for npm dist-tag channels (@beta, @next, @canary) in auto-update mechanism. Users pinned to oh-my-opencode@beta now correctly fetch and compare against beta channel instead of stable latest.
- Add extractChannel() to detect channel from version string
- Modify getLatestVersion() to accept channel parameter
- Update auto-update flow to use channel-aware fetching
- Add comprehensive tests for channel detection and fetching
- Resolves#687
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
