refactor: Move llms.txt generation to follow react.dev patterns

- Create src/utils/llms.js following src/utils/rss.js pattern
- Refactor scripts/generate-llms-txt.js to thin wrapper
- Remove llms-full.txt (simplify to single file approach)
- Add dual copyright headers matching codebase style
- Extract helper functions for cleaner organization

Generates llms.txt (13KB, 167 links) covering 94% of sidebar entries.
Missing 10 files are section index pages with path mismatches.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: artimath

.yarnrc.yml

@@ -0,0 +1 @@
+nodeLinker: node-modules

package.json

@@ -90,7 +90,9 @@
     "postcss-preset-env": "^6.7.0",
     "prettier": "^2.5.1",
     "reading-time": "^1.2.0",
-    "remark": "^12.0.1",
+    "remark": "^15.0.1",
+    "remark-mdx": "^3.1.0",
+    "remark-stringify": "^11.0.0",
     "remark-external-links": "^7.0.0",
     "remark-html": "^12.0.0",
     "remark-images": "^2.0.0",
@@ -117,4 +119,4 @@
     "src/**/*.md": "yarn fix-headings"
   },
   "packageManager": "[email protected]"
-}
+}