Durability pass: tests, clearer diversity/classify behavior, Calm Filters foundation

- Add pytest suite (34 tests) covering scoring thresholds, dedup clustering +
  representative selection + time window, brief source/category diversity,
  avoid-term phrase matching, and text canonicalization/truncation.
- Rewrite _select_diverse with an explicit, tested contract (best-first, one
  per source, backfill, then inject a second category by evicting the
  lowest-ranked pick).
- classify_articles now returns attempted/succeeded/skipped (ClassifyReport) so
  silent model failures are visible in both the cycle and classify output.
- Fix clean_text truncation to stay within max_len (ellipsis no longer
  overshoots).
- New filters.py: canonical FilterPrefs shape (include/mute topics+flavors,
  avoid_terms, pauses) and pure word/phrase-boundary matching engine seeding
  Calm Filters. Not yet wired into the API.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_scoring.py

@@ -0,0 +1,48 @@
+from goodnews.scoring import score_article
+
+
+def test_constructive_story_is_accepted():
+    s = score_article("Community volunteers restore creek habitat", "A hopeful recovery effort", 3)
+    assert s["accepted"] == 1
+    assert s["constructive_score"] >= 5
+    assert s["reason_code"] == "heuristic_constructive_candidate"
+
+
+def test_neutral_story_needs_review():
+    s = score_article("The weather report for tomorrow", None, 3)
+    assert s["accepted"] == 0
+    assert s["reason_code"] == "heuristic_needs_review"
+
+
+def test_cortisol_heavy_is_rejected():
+    s = score_article("War and death as murder and attack escalate", None, 3)
+    assert s["accepted"] == 0
+    assert s["cortisol_score"] > 5
+    assert s["reason_code"] == "heuristic_reject_cortisol_heavy"
+
+
+def test_ragebait_is_rejected_before_cortisol():
+    s = score_article("Senator slams rival and sparks backlash", None, 3)
+    assert s["accepted"] == 0
+    assert s["ragebait_score"] > 3
+    assert s["reason_code"] == "heuristic_reject_ragebait_language"
+
+
+def test_pr_risk_from_source_and_terms_rejects():
+    s = score_article("Startup announces funding round and unveils brand", None, 6)
+    assert s["pr_risk_score"] > 7
+    assert s["accepted"] == 0
+
+
+def test_all_scores_within_bounds():
+    s = score_article("breakthrough cure restores hope " * 10, "progress " * 20, 3)
+    for key in (
+        "constructive_score",
+        "cortisol_score",
+        "ragebait_score",
+        "agency_score",
+        "human_benefit_score",
+        "novelty_score",
+        "pr_risk_score",
+    ):
+        assert 0 <= s[key] <= 10, key