at_mentioned support for sending context (region) to Claude Code

I was looking through https://github.com/coder/claudecode.nvim/blob/main/PROTOCOL.md and discovered the at_mentioned feature.

I have found that Claude Code has difficulty using the current tracking of what is selected.

The mention feature that insert a string into the prompt works better. I have made a pull request 

Pull request : https://github.com/stevemolitor/monet/pull/29