cmd/git(feat[GitStash]): add GitStashManager and GitStashEntryCmd classes

why: Enable programmatic stash management following Manager/Cmd pattern
what:
- Add GitStashEntryCmd for per-stash operations: show, apply, pop, drop, create_branch
- Add GitStashManager for collection-level operations: push, clear, ls, get, filter
- Parse stash list output to create GitStashEntryCmd instances
- Expose git.stashes on Git class (git.stash kept for backwards compatibility)
- CLI flag mappings: -m (message), -p (patch), -S (staged), -k (keep-index), -u (include-untracked)

Author: tony