[ai] fix locations in the skills.

[sayakpaul]

I don't think the agents are deterministic in reading the top-level files we put under .ai. This is also probably why we see divergences from what we explicitly specify in our skills.

Easy verification:

On main, run

$ grep -n '](models.md)' CLAUDE.md
30:- **Models** — see [models.md](models.md) for model conventions, …

$ ls models.md
ls: models.md: No such file or directory

And then on this branch, run:

$ grep -n '\*\*Models\*\*' CLAUDE.md
36:- **Models** — see `.ai/models.md` for model conventions, …


$ ls .ai/models.md
.ai/models.md