cmd/git(feat[GitReflogManager]): add GitReflogManager and GitReflogEntryCmd classes

why: Enable reflog management with Manager/Cmd pattern for consistency
what:
- Add GitReflogEntry dataclass for structured reflog data
- Add GitReflogEntryCmd for per-entry operations (show, delete)
- Add GitReflogManager for collection operations (show, expire, exists)
- Support QueryList-based filtering with ls(), get(), filter() methods
- Parse `git reflog show` output into GitReflogEntry objects
- Add self.reflog to Git class for easy access via git.reflog
- Include comprehensive doctests for all methods

Author: tony