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>

notes/future.md

@@ -1,58 +1,3 @@
-## Milestone 4: Synchronization and Remote Interaction
-
-### Rebase Engine
-
-34. **Implement stacked rebase** (`internal/git/rebase.go`)
-    ```go
-    func RebaseStack(repo *Repository, stack []string, 
-        onto string) error {
-        // 1. Rebase first commit onto target
-        // 2. For each subsequent commit:
-        //    - Rebase onto previous
-        //    - Handle conflicts
-        // 3. Update all branch refs
-    }
-    ```
-
-35. **Integrate rerere** (`internal/git/rerere.go`)
-    - Configure rerere cache location (.onx/rerere_cache)
-    - Enable rerere for rebase operations
-    - Implement conflict detection
-    - Apply recorded resolutions automatically
-
-36. **Create conflict resolution UI** (`internal/git/conflicts.go`)
-    - Detect merge conflicts
-    - Present clear conflict markers
-    - Guide user through resolution
-    - Record resolution for rerere
-
-### Sync and Push Commands
-
-37. **Implement onx sync** (`internal/commands/sync.go`)
-    ```go
-    func Sync(repo *Repository) error {
-        // 1. Begin oplog transaction
-        // 2. Fetch from origin
-        // 3. Get workstream stack
-        // 4. Sequential rebase with rerere
-        // 5. Handle conflicts
-        // 6. Update workstreams.json
-        // 7. Finalize oplog transaction
-    }
-    ```
-
-38. **Implement onx push** (`internal/commands/push.go`)
-    - Get all branches in current workstream
-    - Push each branch to remote
-    - Handle authentication (SSH/HTTPS)
-    - Report progress for each branch
-
-39. **Add remote configuration**
-    - Read git remote configuration
-    - Support multiple remotes
-    - Default to "origin"
-    - Validate remote existence
-
 ## Testing Infrastructure
 
 40. **Create test utilities** (`internal/testutil/`)