Add a centralized ui module with Arc's visual identity: colored commit
IDs (magenta), bookmarks (cyan), tags (yellow), status symbols, and
diff highlighting. Update all command output and tests accordingly.
- Add reset command to discard worktree changes (all or specific files)
- Add revert command to create inverse commits (preserving immutability)
- Add merge command with three-way merge and conflict detection
- Add graft command to cherry-pick commits onto bookmarks
- Create merge.rs with file-level and line-level three-way merge engine
- Create modify.rs orchestrating undo/modification operations
- Add MergeConflicts and NoMergeBase error variants
- Make refs utility functions (write_tree, clean_tracked_files) public
- Make inspect::myers_diff public for reuse in merge engine
- Add comprehensive tests for all four commands
