Closes AI-291

This PR adds some new menu items in the agent panel's ellipsis menu to
make it easier to 1) create a new skill, 2) manage all existing skills
through the settings window, 3) open project and global AGENTS.md file,
if it exists.

Minor UI note: I'm adding an ellipsis character to the menu items in
which you need to do something else in a different view to fulfill the
item's label. For example, "Create Skill…" takes the ellipsis because
clicking on the menu item itself doesn't create the skill, but it takes
you to the place where you'll do that.

<img width="600" alt="Screenshot 2026-05-21 at 10  32@2x"
src="https://github.com/user-attachments/assets/1bfdd423-bd75-4c62-97ba-84130035ede8"
/>

Release Notes:

- N/A