cmd/_async(feat[AsyncGit]): Add async git command class

why: Phase 3 of asyncio support - AsyncGit enables non-blocking git
operations for concurrent workflows.

what:
- Add AsyncGit class with core git commands:
  - run(), clone(), fetch(), checkout(), status()
  - rev_parse(), symbolic_ref(), rev_list(), show_ref()
  - reset(), rebase(), version()
- Add AsyncGitSubmoduleCmd for submodule init/update
- Add AsyncGitRemoteManager for remote ls/show/add/remove/get_url
- Add AsyncGitStashCmd for stash ls/save/pop/drop/clear
- Add comprehensive tests (26 tests) with concurrency tests
- All methods support timeout parameter for cancellation

Author: tony

src/libvcs/cmd/_async/__init__.py

@@ -0,0 +1,17 @@
+"""Async command abstractions for VCS operations.
+
+This module provides async equivalents of the sync command classes
+in :mod:`libvcs.cmd`.
+
+Note
+----
+This is an internal API not covered by versioning policy.
+"""
+
+from __future__ import annotations
+
+from libvcs.cmd._async.git import AsyncGit
+
+__all__ = [
+    "AsyncGit",
+]