Closes AI-230

This PR makes skills, added as /-mentions, be rendered in the agent
panel as creases, like anything you'd @-mention. Naturally, clicking on
the crease button opens the corresponding skill file in a buffer.

It turned out to be quite a bit of plumbing to make this work,
particularly as I am also introducing an interface to display dividers
and headers in the completion menu. This was relevant to me to add
because it sets a good foundation to convert many agent panel-related
actions as slash commands.

Release Notes:

- N/A

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Co-authored-by: MartinYe1234 <52641447+MartinYe1234@users.noreply.github.com>