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_briefs.py

@@ -0,0 +1,50 @@
+from goodnews.briefs import _select_diverse
+
+
+def row(id, source, category):
+    # _select_diverse only reads these three keys; plain dicts support [] access.
+    return {"id": id, "source_name": source, "default_category": category}
+
+
+def test_prefers_distinct_sources_best_first():
+    rows = [
+        row(1, "A", "science"),
+        row(2, "A", "science"),   # same source as #1 — should be skipped while others remain
+        row(3, "B", "science"),
+        row(4, "C", "environment"),
+    ]
+    selected = _select_diverse(rows, limit=3)
+    ids = [r["id"] for r in selected]
+    assert ids == [1, 3, 4]  # one per source, ranked order preserved
+
+
+def test_backfills_when_sources_exhausted():
+    rows = [row(1, "A", "science"), row(2, "A", "science"), row(3, "A", "science")]
+    selected = _select_diverse(rows, limit=2)
+    assert len(selected) == 2  # repeats source A only because no others exist
+
+
+def test_injects_second_category_without_shrinking():
+    rows = [
+        row(1, "A", "science"),
+        row(2, "B", "science"),
+        row(3, "C", "science"),
+        row(4, "D", "environment"),  # the only other category, lowest ranked
+    ]
+    selected = _select_diverse(rows, limit=3)
+    cats = {r["default_category"] for r in selected}
+    assert len(selected) == 3
+    assert len(cats) >= 2  # environment swapped in for diversity
+    assert any(r["default_category"] == "environment" for r in selected)
+
+
+def test_keeps_single_category_when_no_alternative_exists():
+    rows = [row(1, "A", "science"), row(2, "B", "science"), row(3, "C", "science")]
+    selected = _select_diverse(rows, limit=3)
+    assert len(selected) == 3
+    assert {r["default_category"] for r in selected} == {"science"}
+
+
+def test_never_returns_more_than_limit():
+    rows = [row(i, f"S{i}", "science") for i in range(10)]
+    assert len(_select_diverse(rows, limit=5)) == 5