feat: improve LLM discoverability with llms.txt, updated llms-full.txt, and AI crawler rules

- Rewrite llms-full.txt to match current docs (correct tiers, pricing, base URL, all endpoints)
- Add llms.txt index file pointing to llms-full.txt, openapi.json, and key doc pages
- Update robots.txt to explicitly allow AI crawlers (GPTBot, ClaudeBot, PerplexityBot, etc.)

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

Author: Mlaz-code