Closes AI-159
Closes AI-434
Closes AI-435

Release Notes:

- Key agent-related settings now live in the settings editor, close to
all other settings available in Zed. This specifically includes the move
of LLM providers, external agents, and MCP servers to the settings
editor.

---------

Co-authored-by: zed-zippy[bot] <234243425+zed-zippy[bot]@users.noreply.github.com>
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>