Yup with simple AGENTS file and skills, tools like ruler/lnai might be an overkill. However I still think that they are needed for MCPs/Permissions/sub-dir rules/different skills formats.

I would really like all AI agents coding tools to have the same config formats, but I feel like we are not there yet :/

Yes, for MCP servers there's still no good standard. Ruler helps with that. I happen to not use MCPs much, but for a setup that is MCP-heavy that can help.