Add cursor and codex rails ai rules

[gokcelb]

Before, we only had Rails AI rules for Claude. This PR adds them for Codex and Cursor as well.

[jaredlt]

I can see this getting pretty gnarly to manage as we iteratively make updates. Should we look to make it generic?

Differences I see that we may need to validate:

• CLAUDE.md renamed verbatim to AGENTS.md for Codex - will it follow the rules links that the CLAUDE file specifies? I see Codex has something called rules which is different, but I do wonder if it would "just work"?
• Cursor files are .mdc? Again, would it just work if we referenced the .md files or does it absolutely need to be mdc?

[gokcelb]

I can see this getting pretty gnarly to manage as we iteratively make updates. Should we look to make it generic?

Differences I see that we may need to validate:

• CLAUDE.md renamed verbatim to AGENTS.md for Codex - will it follow the rules links that the CLAUDE file specifies? I see Codex has something called rules which is different, but I do wonder if it would "just work"?
• Cursor files are .mdc? Again, would it just work if we referenced the .md files or does it absolutely need to be mdc?

Thanks for raising this. I've been researching about this, and we have a few options.

There's an option to just use an AGENTS.md file and referencing .md files to specific rules. But as I've read online, this is not good for discoverability. The agent doesn't reliably read the rules in this setup.

There's also an in-between option where we could have a shared ai-rules folder (or some other naming) and the "native" agent files, like we have them now, would symlink to these core ai documents. I've read that this gives native-like discoverability, while reducing the maintenance burden (although it doesn't completely eliminate it). I'm researching more about this option currently.