fix: correct main repo root detection for submodules (#849)

internal/git/worktree_test.go

@@ -894,6 +894,54 @@ func TestGetMainRepoRoot(t *testing.T) {
 		}
 	})
 
+	t.Run("returns correct root for submodule repo", func(t *testing.T) {
+		ResetCaches() // Reset caches from previous subtests
+		superRepoPath, superCleanup := setupTestRepo(t)
+		defer superCleanup()
+
+		submoduleRepoPath, submoduleCleanup := setupTestRepo(t)
+		defer submoduleCleanup()
+
+		addCmd := exec.Command("git", "-c", "protocol.file.allow=always", "submodule", "add", submoduleRepoPath, "core")
+		addCmd.Dir = superRepoPath
+		if output, err := addCmd.CombinedOutput(); err != nil {
+			t.Fatalf("Failed to add submodule: %v\nOutput: %s", err, string(output))
+		}
+
+		commitCmd := exec.Command("git", "commit", "-m", "Add submodule")
+		commitCmd.Dir = superRepoPath
+		if output, err := commitCmd.CombinedOutput(); err != nil {
+			t.Fatalf("Failed to commit submodule: %v\nOutput: %s", err, string(output))
+		}
+
+		submodulePath := filepath.Join(superRepoPath, "core")
+
+		// Save current dir and change to submodule
+		originalDir, err := os.Getwd()
+		if err != nil {
+			t.Fatalf("Failed to get current dir: %v", err)
+		}
+		defer func() { _ = os.Chdir(originalDir) }()
+
+		if err := os.Chdir(submodulePath); err != nil {
+			t.Fatalf("Failed to chdir to submodule: %v", err)
+		}
+		ResetCaches() // Reset after chdir
+
+		root, err := GetMainRepoRoot()
+		if err != nil {
+			t.Fatalf("GetMainRepoRoot failed: %v", err)
+		}
+
+		// Resolve symlinks for comparison (e.g., /tmp -> /private/tmp on macOS)
+		expectedRoot, _ := filepath.EvalSymlinks(submodulePath)
+		actualRoot, _ := filepath.EvalSymlinks(root)
+
+		if actualRoot != expectedRoot {
+			t.Errorf("GetMainRepoRoot() = %s, want %s (submodule repo)", actualRoot, expectedRoot)
+		}
+	})
+
 	t.Run("returns main repo root from worktree", func(t *testing.T) {
 		ResetCaches() // Reset caches from previous subtests
 		repoPath, cleanup := setupTestRepo(t)