AGENTS.md is extremely important - it's probably the highest leverage thing you can give your agent. It's injected into every turn, and the agents are trained to follow instructions. If anything, I think people are under-investing into AGENTS.md and going purely based on vibes.
For example, if I write a bad AGENTS.md for a repo with 100 engineers actively working in it, then every agent for every engineer gets worse, without anyone really noticing.
I think we should move towards data-based tuning of AGENTS.md, testing out changes, gathering data, and then making a decision on whether or not to ship it.