Switch to anyhow for improved error handling

- Replace Box<dyn std::error::Error> with anyhow::Result throughout
- Add comprehensive assert_cmd documentation to CLAUDE.md
- Create detailed improvement plan in IMPROVEMENT_PLAN.md
- Use anyhow::anyhow\! for custom error messages
- Add Context trait for better error context where applicable

This provides better error messages and more idiomatic Rust error handling.

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

Co-Authored-By: Claude <[email protected]>

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