fix(ui): stop treating git worktrees and submodules as repos

.git as a file (worktrees, submodules) isn't a standalone repo — the
scanner was flooding the import list with worktree entries alongside
their real repo.

src/ClaudeDo.Ui/Services/RepoScanner.cs

@@ -67,9 +67,7 @@ public static class RepoScanner
         }
     }
 
-    private static bool IsRepo(string dir)
-    {
-        var gitPath = Path.Combine(dir, ".git");
-        return Directory.Exists(gitPath) || File.Exists(gitPath);
-    }
+    // A real clone has .git as a directory. Worktrees and submodules have it as a file
+    // ("gitdir: ...") — those are not standalone repos and would flood the list.
+    private static bool IsRepo(string dir) => Directory.Exists(Path.Combine(dir, ".git"));
 }