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.

[rhardouin]

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