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 commit command with auto-change detection (no staging area)
- Add status command showing added/modified/deleted files
- Add diff command with unified diff output
- Store commits as ZSTD-compressed MessagePack in .arc/commits/<id>.zst
- Generate commit/delta IDs via SHA-256 hashing
- Reconstruct committed state by replaying delta chains
- Add ignore pattern matching with glob support (.arcignore/.ignore)
- Always exclude .arc/ directory from worktree scanning
- Update HEAD and bookmark refs on commit
- Add NothingToCommit error variant
- Add zstd, sha2, hex dependencies
- Add 30 new tests across commit, status, diff, and tracking
