Implement Milestone 4: Synchronization and Remote Interaction

This milestone adds comprehensive remote synchronization and stacked-diff
publishing capabilities to Onyx.

## New Features

### Rebase Engine (internal/git/rebase.go)
- Stacked rebase with sequential commit rebasing
- Conflict detection and handling
- Integration with rerere for automatic conflict resolution
- Support for rebase continuation and abort operations

### Rerere Integration (internal/git/rerere.go)
- Conflict resolution recording and replay
- Cache location: .onx/rerere_cache
- Automatic application of previous conflict resolutions
- Normalized conflict pattern matching via SHA1 hashing

### Conflict Resolution UI (internal/git/conflicts.go)
- Index-based conflict detection (stage 1/2/3)
- Clear conflict presentation with file paths and hashes
- User-friendly resolution guidance
- Conflict marker extraction and analysis

### Remote Commands

#### onx sync (internal/commands/sync.go)
- Fetch latest changes from remote
- Rebase workstream stack onto updated base branch
- Automatic rerere conflict resolution
- Transaction-based with full undo support
- Progress reporting and clear error messages

#### onx push (internal/commands/push.go)
- Push all workstream branches to remote
- Support for force push operations
- Per-branch progress reporting
- Clear summary of pushed branches

### Remote Helpers (internal/git/remote.go)
- Remote validation and configuration
- Support for multiple remotes with origin default
- URL retrieval and remote existence checking

## Implementation Details

- All operations wrapped in oplog transactions for undo support
- Comprehensive error handling and user feedback
- Integration with existing workstream management
- CLI commands registered in cmd/onx/main.go

## Status

Milestone 4 is now complete. All core synchronization and remote
interaction features are implemented and tested.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/onx/main.go

@@ -29,6 +29,8 @@ log for universal undo functionality.`,
 	rootCmd.AddCommand(commands.NewNewCmd())
 	rootCmd.AddCommand(commands.NewListCmd())
 	rootCmd.AddCommand(commands.NewSwitchCmd())
+	rootCmd.AddCommand(commands.NewSyncCmd())
+	rootCmd.AddCommand(commands.NewPushCmd())
 
 	// Execute the root command
 	if err := rootCmd.Execute(); err != nil {