feat(docs): add opt-in llms-full.txt generator + API + dev watcher

[bllfoad]

• Generates a readable LLM corpus from content/*.md|.mdc into public/llms-full.txt
• Preserves code/HTML; converts links/images to text; normalizes whitespace; deterministic order with title + source
• Adds GET /api/llms-full and POST /api/llms-full (token optional via LLMS_FULL_TOKEN)
• Dev watcher regenerates on content changes (only when LLMS_FULL_ENABLE=true)
• Build/dev hooks run generation when enabled; new script: pnpm llms:generate