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 named stash support with create, use, push, pop, rm, and list
commands. Stashes are stored as YAML in .arc/stashes/named/ with a
stack-based push/pop model. Active stash tracked in state.yml.
- stash push saves dirty changes and resets worktree to clean state
- stash pop requires clean worktree and matching HEAD base commit
- 28 integration tests covering all commands and edge cases
- new error variants: StashAlreadyExists, StashNotFound, NoActiveStash,
NothingToStash, StashEmpty, StashBaseMismatch
