docs(internals): Add async_run and async_subprocess docs

why: Document internal async APIs alongside sync equivalents
what:
- Create async_run.md with automodule reference
- Create async_subprocess.md with automodule reference
- Update internals/index.md toctree

Author: tony

docs/internals/async_run.md

@@ -0,0 +1,8 @@
+# async_run - `libvcs._internal.async_run`
+
+Async equivalent of {mod}`libvcs._internal.run`.
+
+```{eval-rst}
+.. automodule:: libvcs._internal.async_run
+   :members:
+```

docs/internals/async_subprocess.md

@@ -0,0 +1,8 @@
+# AsyncSubprocessCommand - `libvcs._internal.async_subprocess`
+
+Async equivalent of {mod}`libvcs._internal.subprocess`.
+
+```{eval-rst}
+.. automodule:: libvcs._internal.async_subprocess
+   :members:
+```

docs/internals/index.md

@@ -14,6 +14,8 @@ types
 dataclasses
 query_list
 run
+async_run
 subprocess
+async_subprocess
 shortcuts
 ```