Simplify architecture diagram in README (#244)

## Summary
Simplified the ASCII architecture diagram in the README to make it more
concise and easier to read while maintaining the same conceptual
information.

## Changes
- Reduced diagram width and complexity by removing excessive spacing and
alignment characters
- Condensed the flow arrows from multi-line to single-line
representation
- Moved the feature list (Sessions, OAuth, Tools, Resources, Prompts,
Tasks, x402) below the diagram for better readability
- Maintained all original information while improving visual clarity

## Details
The diagram now uses a more compact layout that is easier to parse at a
glance, while still clearly showing the relationship between the AI
agent, `mcpc`, and the MCP server. This improves the README's
presentation without changing any technical content or documentation.

https://claude.ai/code/session_013Fze4keWkHv52j43eoS97b

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Co-authored-by: Claude <noreply@anthropic.com>