libvcs(feat[__init__]): Export SyncResult and SyncError from top-level package

why: Users currently must import from libvcs.sync.base directly.
Exporting from __init__.py makes the public API explicit.
what:
- Add SyncResult and SyncError to libvcs.__init__ imports
- Add both to __all__

Author: tony

src/libvcs/__init__.py

@@ -5,7 +5,7 @@
 import logging
 
 from ._internal.run import CmdLoggingAdapter
-from .sync.base import BaseSync
+from .sync.base import BaseSync, SyncError, SyncResult
 from .sync.git import GitSync
 from .sync.hg import HgSync
 from .sync.svn import SvnSync
@@ -16,6 +16,8 @@
     "GitSync",
     "HgSync",
     "SvnSync",
+    "SyncError",
+    "SyncResult",
 ]
 
 logger = logging.getLogger(__name__)