Add key coding principle about error handling in tools

Document that all errors in tools must be thrown as exceptions
rather than returned as strings. This ensures proper error
propagation and handling by the Letta framework.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[e-mail]>

CLAUDE.md

···
 - atproto: Bluesky/AT Protocol integration
 - python-dotenv: Environment management
 - rich: Enhanced terminal output
-- pyyaml: YAML processing
+- pyyaml: YAML processing
+
+## Key Coding Principles
+
+- All errors in tools must be thrown, not returned as strings.