test(v1.4.0-p2): add failing tests for milestones and columns

RED phase: 5 tests in test_collector.py (collect_milestones),
10 tests in test_display.py (render_milestones, parse_columns),
2 tests in test_exporter.py (milestones JSON), 7 tests in
test_cli.py (--milestones, --columns).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/test_exporter.py

@@ -138,3 +138,31 @@ class TestExportJson:
         """Empty repo list produces '[]'."""
         output = export_json([])
         assert json.loads(output) == []
+
+
+class TestExportMilestonesJson:
+    """Test milestones JSON export."""
+
+    def test_export_milestones_json_basic(self):
+        """MilestoneData list produces valid JSON."""
+        from gitea_dashboard.collector import MilestoneData
+        from gitea_dashboard.exporter import export_milestones_json
+
+        milestones = [
+            MilestoneData("repo", "v1.0", 2, 8, 80, "2026-04-01T00:00:00Z", "open"),
+        ]
+
+        output = export_milestones_json(milestones)
+        parsed = json.loads(output)
+
+        assert len(parsed) == 1
+        assert parsed[0]["repo_name"] == "repo"
+        assert parsed[0]["title"] == "v1.0"
+        assert parsed[0]["progress_pct"] == 80
+
+    def test_export_milestones_json_empty(self):
+        """Empty milestone list produces '[]'."""
+        from gitea_dashboard.exporter import export_milestones_json
+
+        output = export_milestones_json([])
+        assert json.loads(output) == []