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>

internal/git/remote.go

@@ -0,0 +1,106 @@
+package git
+
+import (
+	"fmt"
+
+	"github.com/go-git/go-git/v5"
+	"github.com/go-git/go-git/v5/config"
+)
+
+// RemoteHelper provides utilities for working with Git remotes
+type RemoteHelper struct {
+	repo *git.Repository
+}
+
+// NewRemoteHelper creates a new RemoteHelper instance
+func NewRemoteHelper(repo *git.Repository) *RemoteHelper {
+	return &RemoteHelper{repo: repo}
+}
+
+// GetRemote retrieves a remote by name, defaults to "origin" if name is empty
+func (rh *RemoteHelper) GetRemote(name string) (*git.Remote, error) {
+	if name == "" {
+		name = "origin"
+	}
+
+	remote, err := rh.repo.Remote(name)
+	if err != nil {
+		return nil, fmt.Errorf("remote '%s' not found: %w", name, err)
+	}
+
+	return remote, nil
+}
+
+// ListRemotes returns all configured remotes
+func (rh *RemoteHelper) ListRemotes() ([]*git.Remote, error) {
+	remotes, err := rh.repo.Remotes()
+	if err != nil {
+		return nil, fmt.Errorf("failed to list remotes: %w", err)
+	}
+
+	return remotes, nil
+}
+
+// ValidateRemote checks if a remote exists and is properly configured
+func (rh *RemoteHelper) ValidateRemote(name string) error {
+	if name == "" {
+		name = "origin"
+	}
+
+	remote, err := rh.GetRemote(name)
+	if err != nil {
+		return err
+	}
+
+	// Check if remote has URLs configured
+	cfg := remote.Config()
+	if len(cfg.URLs) == 0 {
+		return fmt.Errorf("remote '%s' has no URLs configured", name)
+	}
+
+	return nil
+}
+
+// GetDefaultRemoteName returns the default remote name (origin)
+func (rh *RemoteHelper) GetDefaultRemoteName() string {
+	return "origin"
+}
+
+// GetRemoteURL returns the fetch URL for a remote
+func (rh *RemoteHelper) GetRemoteURL(name string) (string, error) {
+	if name == "" {
+		name = "origin"
+	}
+
+	remote, err := rh.GetRemote(name)
+	if err != nil {
+		return "", err
+	}
+
+	cfg := remote.Config()
+	if len(cfg.URLs) == 0 {
+		return "", fmt.Errorf("remote '%s' has no URLs configured", name)
+	}
+
+	return cfg.URLs[0], nil
+}
+
+// GetRemoteConfig returns the configuration for a remote
+func (rh *RemoteHelper) GetRemoteConfig(name string) (*config.RemoteConfig, error) {
+	if name == "" {
+		name = "origin"
+	}
+
+	remote, err := rh.GetRemote(name)
+	if err != nil {
+		return nil, err
+	}
+
+	return remote.Config(), nil
+}
+
+// HasRemote checks if a remote with the given name exists
+func (rh *RemoteHelper) HasRemote(name string) bool {
+	_, err := rh.repo.Remote(name)
+	return err == nil
+}