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
- Add src/refs.rs with mark_add/rm/list/rename, tag_add/rm/list, switch
- Bookmarks stored as YAML RefTarget files in .arc/bookmarks/
- Tags stored as YAML RefTarget files in .arc/tags/ (immutable)
- Switch reconstructs worktree from delta chain, checks for dirty state
- Add error variants: BookmarkNotFound, BookmarkAlreadyExists, TagNotFound,
TagAlreadyExists, CannotRemoveActiveMark, DirtyWorktree
- Wire up CLI dispatch for mark, tag, and switch commands
- Fix cli.rs tests to run mark/tag list within a repo context
- Add 35 new tests across tests/mark.rs, tests/tag.rs, tests/switch.rs
