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>

goodnews/text.py

@@ -26,7 +26,8 @@ def clean_text(value: str | None, max_len: int = 1000) -> str | None:
     text = html.unescape(text)
     text = WHITESPACE_RE.sub(" ", text).strip()
     if len(text) > max_len:
-        return text[: max_len - 1].rstrip() + "..."
+        # Keep the ellipsis inside max_len rather than overshooting by 3.
+        return text[: max_len - 3].rstrip() + "..."
     return text or None