Implement a proper error handling strategy across the codebase:

1. Define custom error types using thiserror in each module
2. Implement From conversions for error propagation
3. Replace anyhow with typed errors in library code (keep anyhow in CLI)
4. Add error codes for programmatic error handling
5. Ensure all errors have useful context messages

This improves debuggability and allows callers to handle specific error cases.