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>

docs/comparison-results/json/diagnose-baseline.json

@@ -0,0 +1,14 @@
+{
+  "openai": true,
+  "xai": true,
+  "x_source": "bird",
+  "bird_installed": true,
+  "bird_authenticated": true,
+  "bird_username": "env AUTH_TOKEN",
+  "youtube": true,
+  "hackernews": true,
+  "web_search_backend": null,
+  "parallel_ai": false,
+  "brave": false,
+  "openrouter": false
+}