chore: enable canvas, disable bookmarks, document single-tier config

Enable the canvas core plugin and disable bookmarks in core-plugins.json.
Rewrite the README to describe the collapsed single-tier global config
(the per-vault fork tier and bookmarks.json are retired) and add a
comprehensive git submodule how-to.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CsXyUHsjzgFfz2EWPrSQ2U

core-plugins.json

@@ -4,7 +4,7 @@
   "switcher": true,
   "graph": true,
   "backlink": true,
-  "canvas": false,
+  "canvas": true,
   "outgoing-link": true,
   "tag-pane": true,
   "footnotes": false,
@@ -16,7 +16,7 @@
   "command-palette": true,
   "slash-command": false,
   "editor-status": true,
-  "bookmarks": true,
+  "bookmarks": false,
   "markdown-importer": false,
   "zk-prefixer": false,
   "random-note": false,