Replace /create-app with /flutter:init for brownfield projects #50
Description
This is a proposal for a /flutter:init command that would initialize the extension in an existing Flutter project. This would solve #21 and #24 by separating out the project-creation step from the initialize-extension-prototyping-experience step.
This would work similarly to /speckit.init, and would also require that we namespace our commands, since there is already a default /init command in Gemini CLI that creates GEMINI.md.