Improve management of Roo config files

[ksze]

Currently, when Roo looks for the clinerules of a particular mode, it looks for <project root>/.clinerules-<slug>. If we have multiple modes, then we have multiple such files at the project root. It's kinda ugly.

I propose to make Roo look for the files under a <project root>/.roo folder first. Only when the <project root>/.roo folder is not found, then Roo looks for the old .clinerules-<slug> files at the project root.

We can structure the folder like this:

• <project root>/.roo folder
  • rules file, which replaces the project-wide .clinerules
  • custom-modes folder
    • multiple <slug> files, which contain YAML to describe:
      • full name
      • role definition
      • tools
      • mode-specific custom instructions

I suppose the above project-specific custom mode config would override the global ones that happen to have the same slug.

Reference discussion on Discord: https://discord.com/channels/1332146336664915968/1332532062124511262/1338210703668150453