Separate /create-app into separate commands

[johnpryan]

In addition to #50, I think we should further separate /create-app into separate commands:

• /flutter:init - initializes the extension with any context or files that are used to drive the agent prototyping experience we want to ship in the extension (Replace /create-app with /flutter:init for brownfield projects #50)
• /flutter:specify <prompt> - creates a DESIGN.md file from a prompt
• /flutter:tasks - creates the IMPLEMENTATION.md file
• /flutter:implement - implements the next phase of the implmentation plan (Have a separate /implement command #48)

[johnpryan]

I also think DESIGN.md and IMPLEMENTATION.md should be placed into a directory dedicated to specifications, similar to SpecKit (#47) but in this proposal I am still assuming the DESIGN.md and IMPLEMENTATION.md behavior we are currently using.