docs: Add CLAUDE.md to improve AI-assisted development with Claude Code

[timdauer]

Upon several requests we would like to create some tooling support for implementation of backends using claude code. Therefore some files kick-starting claude code usage should be added.

Summary

This issue proposes adding CLAUDE.md files to the repository to provide guidance for https://claude.ai/code when working with this codebase.

Motivation

As AI-assisted development tools become more prevalent, providing structured context helps these tools work more effectively with our codebase. Claude Code specifically looks for CLAUDE.md files to
understand:

• Project structure and architecture
• Build, test, and development commands
• Codebase conventions and patterns
• Domain-specific rules and constraints

For an MCP server project, this is particularly relevant since we're building infrastructure for AI-agent interactions - it makes sense to also optimize for AI-assisted development of that
infrastructure.

Proposed Changes

1. Root CLAUDE.md - Project overview, structure, development commands, and MCP adapter behavioral principles
2. backends/** - Backend mappings that ease integration to various backends in an efficient way

Benefits

• Faster onboarding for contributors using Claude Code
• Consistent adherence to MCP adapter principles (no business logic, transparent error propagation, backend-agnostic behavior)
• Reduced friction when implementing new tools or adapters
• Documentation that serves both humans and AI assistants

Questions for Discussion

1. Are there additional conventions or patterns that should be documented?
2. Should adapter implementations (created from adapter-template/) include their own CLAUDE.md?
3. Any concerns about maintaining these files alongside traditional documentation?

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Feedback welcome on the approach and content.