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.
- config set/get/show/unset commands with dotted key paths (user.name, default.bookmark, aliases.c, etc.)
- local config overrides global config per rule 21
- alias expansion at CLI level before dispatch per rule 23
- InvalidConfigKey error variant
- 28 tests covering all config operations, global/local resolution, and alias expansion