chore: untrack third-party plugins from user repo

All plugins except cache-on-save and story-blocks are managed by GPM
and server-install.sh — they should not be in version control.
Fixes .gitignore to plugins/* with explicit allows for custom plugins only.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vgmzx8VTTTmCskSpQtsLTr

.gitignore

@@ -1,5 +1,6 @@
-/plugins/
-!/plugins/
+/plugins/*
+!/plugins/.gitkeep
 !/plugins/cache-on-save/
+!/plugins/story-blocks/
 /data/
 /pages/01.trips/italy-2026-demo/