Add agent skills for working on ty (#25422)

## Summary

This PR adds a `.agents/skills` directory to the repo, and adds three
skills to help agents work on ty more effectively:
- A skill to help them write better ty diagnostics
- A skill to help them minimize a single ecosystem change more
effectively
- A skill to help them analyze and summarise ecosystem reports more
effectively

The skills have been designed so that they will (hopefully) work well
whether a contributor is using Codex or Claude as their agent locally.
Codex should discover these skills automatically if you're working on
the repo; if you use Claude locally, you _should_ hopefully be prompted
by Claude to install the local skills due to them being listed as
recommended skills in `.claude/settings.json`.

## Test Plan

I ran `/review` locally with Codex, instructing it to be as nitpicky as
possible, until it could find no more issues with these skills.