docs(sync[update_repo]): Document SyncResult return type in README and base class

why: README showed a bare update_repo() call that ignored errors, and
BaseSync.update_repo() docstring lacked a Returns section for autodoc.
what:
- Update README sync example to capture SyncResult and check .ok/.errors
- Add NumPy-style Returns section to BaseSync.update_repo() docstring

Author: tony

README.md

@@ -70,9 +70,13 @@ repo = GitSync(
 )
 
 # Clone (if not exists) or fetch & update (if exists)
-repo.update_repo()
+result = repo.update_repo()
 
-print(f"Current revision: {repo.get_revision()}")
+if result.ok:
+    print(f"Current revision: {repo.get_revision()}")
+else:
+    for error in result.errors:
+        print(f"Sync failed at {error.step}: {error.message}")
 ```
 
 ### 2. Command Abstraction

src/libvcs/sync/base.py

@@ -278,7 +278,13 @@ def ensure_dir(self, *args: t.Any, **kwargs: t.Any) -> bool:
         return True
 
     def update_repo(self, *args: t.Any, **kwargs: t.Any) -> SyncResult:
-        """Pull latest changes to here from remote repository."""
+        """Pull latest changes to here from remote repository.
+
+        Returns
+        -------
+        SyncResult
+            Result of the sync operation, with any errors recorded.
+        """
         raise NotImplementedError
 
     def obtain(self, *args: t.Any, **kwargs: t.Any) -> None: