security: remove skool-hub private assets from repo

Removes all 41 skool-hub images from tracking, cleans all canvas
and workspace.json references. Adds _attachments/images/skool-hub/
to .gitignore. History purge follows.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

.obsidian/workspace.json

@@ -255,8 +255,6 @@
     "_attachments/the-frontier.png",
     "_attachments/neural-voyager.png",
     "_attachments/code-genesis.png",
-    "_attachments/images/skool-hub/apr-slot2-results.png",
-    "_attachments/images/skool-hub/apr-bg-v05.png",
     "wiki/concepts/Hot Cache.md",
     "wiki/concepts/Compounding Knowledge.md",
     "wiki/entities/_index.md",