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_smoke.py

@@ -0,0 +1,99 @@
+"""End-to-end smoke tests — run the actual script as subprocess."""
+
+import json
+import subprocess
+import sys
+import unittest
+from pathlib import Path
+
+SCRIPT = str(Path(__file__).parent.parent / "scripts" / "last30days.py")
+
+
+def _run(args, timeout=30):
+    """Run last30days.py with args, return (returncode, stdout, stderr)."""
+    result = subprocess.run(
+        [sys.executable, SCRIPT] + args,
+        capture_output=True, text=True, timeout=timeout,
+    )
+    return result.returncode, result.stdout, result.stderr
+
+
+class TestDiagnose(unittest.TestCase):
+    """Tests for --diagnose flag."""
+
+    def test_exits_zero(self):
+        rc, stdout, stderr = _run(["--diagnose"])
+        self.assertEqual(rc, 0, f"--diagnose failed: {stderr}")
+
+    def test_returns_valid_json(self):
+        rc, stdout, stderr = _run(["--diagnose"])
+        data = json.loads(stdout)
+        self.assertIsInstance(data, dict)
+
+    def test_has_expected_keys(self):
+        rc, stdout, stderr = _run(["--diagnose"])
+        data = json.loads(stdout)
+        for key in ("openai", "xai", "youtube", "tiktok", "instagram", "hackernews", "polymarket"):
+            self.assertIn(key, data, f"Missing key: {key}")
+
+    def test_boolean_values(self):
+        rc, stdout, stderr = _run(["--diagnose"])
+        data = json.loads(stdout)
+        for key in ("openai", "xai", "youtube", "tiktok", "instagram", "hackernews", "polymarket"):
+            self.assertIsInstance(data[key], bool, f"{key} should be boolean")
+
+    def test_hackernews_always_true(self):
+        data = json.loads(_run(["--diagnose"])[1])
+        self.assertTrue(data["hackernews"])
+
+    def test_polymarket_always_true(self):
+        data = json.loads(_run(["--diagnose"])[1])
+        self.assertTrue(data["polymarket"])
+
+
+class TestHelp(unittest.TestCase):
+    """Tests for --help flag."""
+
+    def test_exits_zero(self):
+        rc, stdout, stderr = _run(["--help"])
+        self.assertEqual(rc, 0)
+
+    def test_shows_usage(self):
+        rc, stdout, stderr = _run(["--help"])
+        self.assertTrue(
+            "topic" in stdout.lower() or "usage" in stdout.lower(),
+            "Expected usage info in help output"
+        )
+
+
+class TestNoTopic(unittest.TestCase):
+    """Tests for missing topic."""
+
+    def test_exits_nonzero_without_topic(self):
+        rc, stdout, stderr = _run([])
+        self.assertNotEqual(rc, 0)
+
+    def test_error_message(self):
+        rc, stdout, stderr = _run([])
+        self.assertTrue(
+            "topic" in stderr.lower() or "error" in stderr.lower(),
+            "Expected error about missing topic"
+        )
+
+
+class TestMockMode(unittest.TestCase):
+    """Tests for --mock mode (fixture-based, no API calls)."""
+
+    def test_mock_json_exits_zero(self):
+        rc, stdout, stderr = _run(["--mock", "--emit", "json", "test topic"], timeout=120)
+        self.assertEqual(rc, 0, f"--mock failed: {stderr}")
+
+    def test_mock_json_valid(self):
+        rc, stdout, stderr = _run(["--mock", "--emit", "json", "test topic"], timeout=120)
+        data = json.loads(stdout)
+        self.assertIn("topic", data)
+        self.assertEqual(data["topic"], "test topic")
+
+
+if __name__ == "__main__":
+    unittest.main()