docs: rewrite README as AI Agent-First specification

- Replace human-oriented README with precise, AI-executable specification
- Remove AGENTS.md (merged into README.md as single source of truth)
- Use IF/THEN/NEVER format for rules, tables for field definitions
- Number all critical rules (0.1-0.7) for cross-referencing
- Add complete minimal examples (JSON API, HTML+JSON, embed.FS, middleware group)
- Consolidate gin/echo/chi differences and "what xun does NOT have"
- Add section and rule indexes for quick lookup

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: cnlangzi