test: improve coverage to >80% on all core packages

Add comprehensive test coverage for internal packages:

- internal/config: 66.2% → 98.5%
- internal/dns: 67.8% → 84.3%
- internal/output: 48.8% → 89.1%
- internal/traverse: 56.3% → 86.9%

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

internal/config/config_test.go

@@ -214,3 +214,76 @@ func TestParseDebugLevel(t *testing.T) {
 		}
 	}
 }
+
+func TestParseMaxDepthValid(t *testing.T) {
+	cases := []struct {
+		input string
+		want  int
+	}{
+		{"1", 1},
+		{"20", 20},
+		{"100", 100},
+	}
+	for _, tc := range cases {
+		got, err := ParseMaxDepth(tc.input)
+		if err != nil {
+			t.Errorf("ParseMaxDepth(%q) unexpected error: %v", tc.input, err)
+		}
+		if got != tc.want {
+			t.Errorf("ParseMaxDepth(%q) = %d, want %d", tc.input, got, tc.want)
+		}
+	}
+}
+
+func TestParseMaxDepthInvalid(t *testing.T) {
+	cases := []string{"0", "101", "notanumber", "-1"}
+	for _, s := range cases {
+		_, err := ParseMaxDepth(s)
+		if err == nil {
+			t.Errorf("ParseMaxDepth(%q): expected error", s)
+			continue
+		}
+		if !errors.Is(err, ErrInvalidMaxDepth) {
+			t.Errorf("ParseMaxDepth(%q): expected ErrInvalidMaxDepth, got %v", s, err)
+		}
+	}
+}
+
+func TestParseRetriesValid(t *testing.T) {
+	cases := []struct {
+		input string
+		want  int
+	}{
+		{"0", 0},
+		{"2", 2},
+		{"10", 10},
+	}
+	for _, tc := range cases {
+		got, err := ParseRetries(tc.input)
+		if err != nil {
+			t.Errorf("ParseRetries(%q) unexpected error: %v", tc.input, err)
+		}
+		if got != tc.want {
+			t.Errorf("ParseRetries(%q) = %d, want %d", tc.input, got, tc.want)
+		}
+	}
+}
+
+func TestParseRetriesInvalid(t *testing.T) {
+	cases := []string{"-1", "11", "notanumber"}
+	for _, s := range cases {
+		_, err := ParseRetries(s)
+		if err == nil {
+			t.Errorf("ParseRetries(%q): expected error", s)
+			continue
+		}
+		if !errors.Is(err, ErrInvalidRetries) {
+			t.Errorf("ParseRetries(%q): expected ErrInvalidRetries, got %v", s, err)
+		}
+	}
+}
+
+func TestPrintUsage(t *testing.T) {
+	// PrintUsage writes to stderr; just ensure it doesn't panic.
+	PrintUsage()
+}