Closes AI-266

This PR adds a built-in skill called `create-skill`, which allows the
Zed agent to have access to a skill that teaches it how to properly
create skills for Zed. You can manually invoke it as well as just
letting the model auto-invoke it in case your prompt suggests creating a
new skill.

Release Notes:

- Agent: Added a built-in skill called `create-skill` to make the Zed
agent informed about how to do that.

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Co-authored-by: Richard Feldman <richard@zed.dev>