Add CLAUDE.md with setup instructions for future agents

Author: madaboutcode

CLAUDE.md

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+# madaboutcode.github.com
+
+Jekyll blog for [madaboutcode.com](https://madaboutcode.com). Jekyll 4.4.1, kramdown, rouge.
+
+## Commands
+
+```bash
+./startserver.sh     # Serves http://localhost:4000
+bundle exec jekyll build  # Production build
+```
+
+## Ruby + Bundler
+
+- System Ruby (2.6.10) is too old — use Homebrew Ruby 3.4.8
+- Path: `/opt/homebrew/Cellar/ruby/3.4.8/bin`
+- Always use `bundle exec jekyll` — bare `jekyll` won't work
+- Gemfile needs `jekyll` and `webrick`
+
+## Build Output
+
+`_site/` is Jekyll build output. Gitignored. Rebuild with `bundle exec jekyll build`.
+
+## Verification
+
+`bundle exec jekyll build` must pass.