Support global AGENTS.md

[xxchan]

Currently we only support project level AGENTS.md. We may add sth like ~/.kimi/AGENTS.md

[xiaods]

I don't think supporting a global AGENTS.md is a good idea; more compelling reasons are needed.

[c31io]

I don't think supporting a global AGENTS.md is a good idea; more compelling reasons are needed.

It's a standard feature these days, that is very useful in feeding host/personal preference. For example, Kimi Code always generates bash command for shell tool first, then realized it is working in a different shell. A global AGENT.md would have informed that.

[rhardouin]

I like user/global AGENTS.md.
@xiaods why do you think it's a bad idea, because rules should be converted to skills?