Update CLAUDE.md with dev commands, 3-site architecture, and codegen details

Keep all existing writing conventions, patterns, and guidelines intact.
Add new sections for:
- Development commands (generate, format, lint, validate-*)
- Three GitBook sites (guides, api-reference, brand-guides)
- Codegen pipeline (smith.ts, Metalsmith, Handlebars, @seamapi packages)
- Auto-generated vs. hand-written content guidance

Fix stale references from /docs/api/ to docs/api-reference/.

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

Author: louis-pre