feat(quality): YouTube relevance scoring and cross-source linking

YouTube videos now get real relevance scores based on token overlap
between the search query and video title (was hardcoded at 0.7).
Uses ratio overlap with stopword removal, floored at 0.1.

Cross-source linking annotates items that discuss the same story
across different platforms (e.g., Reddit + HN + X). Items get
bidirectional cross_refs displayed as [xref: R3, HN5] in compact
output so Claude can triangulate multi-platform coverage.

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

scripts/last30days.py

@@ -1113,6 +1113,11 @@ def main():
         by_relevance = sorted(normalized_reddit, key=lambda item: item.relevance, reverse=True)
         deduped_reddit = by_relevance[:3]
 
+    # Cross-source linking: annotate items that discuss the same story
+    dedupe.cross_source_link(
+        deduped_reddit, deduped_x, deduped_youtube, deduped_hn, deduped_web,
+    )
+
     progress.end_processing()
 
     # Create report