cmd/_async(feat): Add AsyncGit, AsyncHg, AsyncSvn command classes

why: Enable non-blocking VCS command execution for better concurrency.

what:
- Add AsyncGit with async run(), clone(), fetch(), etc.
- Add AsyncHg with async run(), clone(), pull(), etc.
- Add AsyncSvn with async run(), checkout(), update(), etc.

Author: tony

src/libvcs/cmd/_async/__init__.py

@@ -0,0 +1,21 @@
+"""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
+from libvcs.cmd._async.hg import AsyncHg
+from libvcs.cmd._async.svn import AsyncSvn
+
+__all__ = [
+    "AsyncGit",
+    "AsyncHg",
+    "AsyncSvn",
+]