cmd/git(fix[GitRemoteManager.add]): Validate mirror parameter correctly

why: The assert statement was checking `any(f for f in ["push", "fetch"])`
which always returns True since non-empty strings are truthy. The actual
mirror value was never validated.
what:
- Replace always-true assertion with proper validation
- Raise ValueError if mirror is not 'push' or 'fetch'
- Fix --mirror syntax to use = format per git-remote docs
- Simplify bool check with elif

Author: tony

src/libvcs/cmd/git.py

@@ -4265,9 +4265,11 @@ def add(
             local_flags.extend(["-m", master])
         if mirror is not None:
             if isinstance(mirror, str):
-                assert any(f for f in ["push", "fetch"])
-                local_flags.extend(["--mirror", mirror])
-            if isinstance(mirror, bool) and mirror:
+                if mirror not in ("push", "fetch"):
+                    msg = "mirror must be 'push' or 'fetch'"
+                    raise ValueError(msg)
+                local_flags.append(f"--mirror={mirror}")
+            elif mirror is True:
                 local_flags.append("--mirror")
         return self.run(
             "add",