feat(quality): GOAT synthesis improvements - hybrid cross-source linking, YouTube synonyms, human-readable xref tags

Ran 15-way blinded comparison (5 topics x 3 versions). CROSS won all 5 topics
(4.74/5.0 avg vs HN 4.10, Base 3.73). Then improved CROSS further:

- dedupe.py: hybrid similarity (token+trigram Jaccard) at 0.40 threshold,
  cross-source links went from 3 to 13 items across 5 topics
- render.py: [xref: HN5, HN4] -> [also on: HN, Reddit] for human-readable tags
- youtube_yt.py: SYNONYMS dict so "hip hop" matches "rap" (0.33 -> 0.71 score)
- SKILL.md: instruction #7 tells Claude to lead with cross-platform signals

Validation: improved CROSS scores 4.38/5.0 vs original 3.98 (+0.40), wins 4/5
topics. Biggest gains in specificity (+0.8) and format compliance (+1.0).

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

scripts/lib/render.py

@@ -12,10 +12,24 @@ OUTPUT_DIR = Path.home() / ".local" / "share" / "last30days" / "out"
 
 
 def _xref_tag(item) -> str:
-    """Return ' [xref: X1, HN3]' string if item has cross_refs, else ''."""
+    """Return ' [also on: Reddit, HN]' string if item has cross_refs, else ''."""
     refs = getattr(item, 'cross_refs', None)
-    if refs:
-        return f" [xref: {', '.join(refs)}]"
+    if not refs:
+        return ""
+    source_names = set()
+    for ref_id in refs:
+        if ref_id.startswith('R'):
+            source_names.add('Reddit')
+        elif ref_id.startswith('X'):
+            source_names.add('X')
+        elif ref_id.startswith('YT'):
+            source_names.add('YouTube')
+        elif ref_id.startswith('HN'):
+            source_names.add('HN')
+        elif ref_id.startswith('W'):
+            source_names.add('Web')
+    if source_names:
+        return f" [also on: {', '.join(sorted(source_names))}]"
     return ""