Reconsider default agent packages for nvim-sidekick

[jolars]

Should we really default to including all these in the sidekick plugin?

nixvim/plugins/by-name/sidekick/default.nix

Lines 14 to 17 in 21ae25e

	"claude-code"
	"copilot"
	"gemini"
	"opencode"

Isn't it safer to simply default to none and let users decide themselves?