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"));
 }

tests/ClaudeDo.Ui.Tests/RepoScannerTests.cs

@@ -35,15 +35,17 @@ public sealed class RepoScannerTests : IDisposable
     }
 
     [Fact]
-    public void Scan_TreatsDotGitFileAsRepo()
+    public void Scan_IgnoresWorktrees()
     {
-        var repo = MakeDir("worktree-repo");
-        File.WriteAllText(Path.Combine(repo, ".git"), "gitdir: ../somewhere");
+        var repo = MakeDir("repo-a");
+        Directory.CreateDirectory(Path.Combine(repo, ".git"));
+        var worktree = MakeDir("repo-a-my-task");
+        File.WriteAllText(Path.Combine(worktree, ".git"), "gitdir: ../repo-a/.git/worktrees/my-task");
 
         var result = RepoScanner.Scan(_root);
 
         Assert.Single(result);
-        Assert.Equal("worktree-repo", result[0].Name);
+        Assert.Equal("repo-a", result[0].Name);
     }
 
     [Fact]