refactor: wire all chapters to Astro site, simplify navigation

- Remove CODE_OF_CONDUCT, CONTRIBUTING, SANDBOX
- Replace operations subsections and test pages with 9 single-page chapters
- Rename concepts to building-blocks to match canonical chapter
- Consolidate installation instructions under one section
- Replace command subsections with daily operations reference table
- Rewrite exercises as a linear workflow: install, clone, add, commit, push, pull
- Update quiz to test core concepts (index purpose, commit chain)
- Simplify nav.ts — all sections are single-page, no subsections
- Add collapsible ToC with h2 headings, expandable h3+ children
- Disable syntax highlighting — dark terminal style code blocks
- Add assets symlink to .gitignore (Windows compatibility)

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

Author: braboj

.gitignore

@@ -48,3 +48,4 @@ Thumbs.db
 *.mov
 *.wmv
 
+astro-site/src/content/assets