docs: add architecture diagram to README

[Tonyfudecai]

Description

Add an ASCII or Mermaid architecture diagram to README.md showing the internal structure of PicoClaw:

• Message Bus
• Channel system (Telegram, Discord, etc.)
• Agent Loop
• Tool Registry
• Skill system

Acceptance Criteria

• Diagram clearly shows component relationships
• Diagram is rendered correctly on GitHub

Skills needed

Markdown, basic understanding of Go project structure