feat: add YouTube Script Writer skill (experimental) (#50) (#56)

Reapplies @prajwal-28's PR #50 onto current main WITHOUT the LF->CRLF
line-ending conversion the original PR introduced across skill-tiers.json
and the export indexes:
- adds skills/youtube-script-writer/SKILL.md (retention-optimized video scripts)
- registers it in the experimental tier
- regenerates the 5 platform exports with LF endings (no whole-file churn)



Claude-Session: https://claude.ai/code/session_016JWn5jRD5tcEFKrubjQ6Px

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: prajwal-28 <prajwal-28@users.noreply.github.com>

skill-tiers.json

@@ -58,6 +58,7 @@
     "morning-intelligence",
     "last-30-days-research",
     "competitor-signal-tracker",
-    "multi-source-signal-synthesiser"
+    "multi-source-signal-synthesiser",
+    "youtube-script-writer"
   ]
 }