[AI] Settle on some AGENTS.md best practices.

[joshuapease]

Nuggets:

• https://github.com/AdCouncil/smokeybear-craft/pull/364/changes#diff-6ebdb617a8104a7756d0cf36578ab01103dc9f07e4dc6feb751296b9c402faf7
• See: Add Craft CMS Expert skill viget-agent-skills#11