Add Cline provider and Nix flake support

[MostlyKIGuess]

• Add ClineProvider with JSON stream parsing and CLI integration
• Register Cline as a new provider type and agent
• Add Nix flake for reproducible dev environment

[BahaAbuNojaim]

When Cline is not installed and the user clicks on Install then the installation dialog faces few issues:

• Installing command whether manual or automatic is wrong, should be cline's installing command, currently it "ext install saoudrizwan.claude-dev" and it should be "npm install -g cline" as in https://docs.cline.bot/cline-cli/installation
• Logo not appearing
• Should ensure authentication via "cline auth" command
• Should follow CLI commands
• Output format rendered in Mysti should be matching to other providers and well formatted

[MostlyKIGuess]

When Cline is not installed and the user clicks on Install then the installation dialog faces few issues:

* Installing command whether manual or automatic is wrong, should be cline's installing command, currently it "ext install saoudrizwan.claude-dev" and it should be "npm install -g cline" as in https://docs.cline.bot/cline-cli/installation

* Logo not appearing

* Should ensure authentication via "cline auth" command

* Should follow CLI commands

* Output format rendered in Mysti should be matching to other providers and well formatted

Ah, I thought we were supposed to install the VSCode extension, I changed it to cli now and update the dependencies. I am not sure why the logo is an issue, could you compile it again? The cline file is present in resources.