test: add smoke tests and edge case coverage

Add end-to-end smoke tests and expand coverage for score and render modules:

- test_smoke.py (new): subprocess tests for --diagnose, --help, --mock,
  and missing topic. Validates exit codes, JSON structure, and source
  detection (HN/Polymarket always available).
- test_score.py: add TestCommentQualityWeight (top_comment_score boost),
  TestInstagramEngagement (basic scoring, views vs likes weighting)
- test_render.py: add TestEnsureOutputDir, TestXrefTag, TestRenderEmptyReport

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

tests/test_render.py

@@ -112,5 +112,53 @@ class TestGetContextPath(unittest.TestCase):
         self.assertIn("last30days.context.md", result)
 
 
+class TestEnsureOutputDir(unittest.TestCase):
+    """Tests for ensure_output_dir()."""
+
+    def test_creates_directory(self):
+        import os
+        import tempfile
+        test_dir = os.path.join(tempfile.mkdtemp(), "output", "nested")
+        os.environ["LAST30DAYS_OUTPUT_DIR"] = test_dir
+        try:
+            render.ensure_output_dir()
+            self.assertTrue(os.path.exists(test_dir))
+        finally:
+            os.environ.pop("LAST30DAYS_OUTPUT_DIR", None)
+
+
+class TestXrefTag(unittest.TestCase):
+    """Tests for _xref_tag()."""
+
+    def test_no_refs(self):
+        item = schema.RedditItem(id="R1", title="T", url="u", subreddit="s")
+        self.assertEqual(render._xref_tag(item), "")
+
+    def test_with_refs(self):
+        item = schema.RedditItem(
+            id="R1", title="T", url="u", subreddit="s",
+            cross_refs=["X1", "HN2"],
+        )
+        tag = render._xref_tag(item)
+        self.assertIn("X", tag)
+        self.assertIn("HN", tag)
+
+
+class TestRenderEmptyReport(unittest.TestCase):
+    """Test render_compact handles empty reports gracefully."""
+
+    def test_empty_items_graceful(self):
+        report = schema.Report(
+            topic="test",
+            range_from="2026-02-04",
+            range_to="2026-03-06",
+            generated_at="2026-03-06T00:00:00+00:00",
+            mode="both",
+        )
+        output = render.render_compact(report)
+        self.assertIn("test", output)
+        self.assertIsInstance(output, str)
+
+
 if __name__ == "__main__":
     unittest.main()