Add publish workflow with PR creation for Phase 3
- Create branch API client with status and publish endpoints
- Add useBranchStatus and usePublish React Query hooks
- Create PublishDialog component with commit and PR inputs
- Create PublishSuccessDialog showing PR link and commit info
- Integrate publish flow into EditorContainer
- Add Publish button that creates PRs from edited files
- Disable publish button when there are unsaved changes
- Show branch status and edited files in publish dialog
- Display success message with link to created PR