Closes AI-295

This PR adds a skills submenu within the "add context" menu in the agent
panel's message editor. This will hopefully be yet another way to find
skills in the app.

<img width="600" alt="Screenshot 2026-05-21 at 11  24@2x"
src="https://github.com/user-attachments/assets/43652081-3929-4ca0-a32b-464077d84dd5"
/>

Release Notes:

- N/A