chore: update plugin catalog URL to git.fabledsword.com

Default plugins.index_url and all documentation URLs now point to
https://git.fabledsword.com/bvandeusen/FabledScryer-plugins.
Anchor /plugins/ in .gitignore so it only matches the root directory
(not docs/plugins/).

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

.gitignore

@@ -3,7 +3,7 @@ docs/superpowers/
 
 # Plugin directory — managed as a separate repo (bvandeusen/fabledscryer-plugins)
 # PLUGIN_DIR in config.yaml points here at runtime
-plugins/
+/plugins/
 
 # Python
 __pycache__/