dont remove agents silently

[teto]

Opening a follow up of #93 (comment) not to pollute the PR:
in

gp.nvim/lua/gp/init.lua

Line 745 in d76be3d

if type(agent) ~= "table" or not agent.model or not agent.system_prompt then

gp.nvim removes silently agents that dont specify .model agent.system_prompt.

It's not possible to understand without looking at the code, I think it would be better to generate an error during startup, at minimum notify the user in checkhealth gp

[Robitx]