feat: harden workspace file APIs

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/fs/localfs_test.go

@@ -135,3 +135,12 @@ func TestLocalFSListSorted(t *testing.T) {
 		t.Errorf("List not sorted: %v", names)
 	}
 }
+
+func TestLocalFSRejectsRemoveRoot(t *testing.T) {
+	fsys := NewLocal(t.TempDir())
+	for _, path := range []string{"", "."} {
+		if err := fsys.Remove(path); err == nil {
+			t.Fatalf("Remove(%q) expected error", path)
+		}
+	}
+}