cmd/git(feat[GitNotesManager]): add GitNotesManager and GitNoteCmd classes

why: Enable notes management with Manager/Cmd pattern for consistency
what:
- Add GitNoteCmd for per-note operations (show, remove, append, edit, copy)
- Add GitNotesManager for collection operations (add, prune, merge, get_ref)
- Support QueryList-based filtering with ls(), get(), filter() methods
- Parse `git notes list` output into GitNote dataclasses
- Add self.notes to Git class for easy access via git.notes
- Include comprehensive doctests for all methods

Author: tony