Further improve instructions for agents for working on ty (#25430)

## Summary

A bunch of followup changes from
https://github.com/astral-sh/ruff/pull/25422. Mostly triggered by using
the skill for the first time to analyze the report in
https://github.com/astral-sh/ruff/pull/24761.

- Move ty-specific guidance from `AGENTS.md` to a new local
`working-on-ty` skill. Link to the other ty-specific skills from that
skill.
- Add more keyword triggers to various skills so that agents are more
likely to automatically figure out that a skill is appropriate
(suggested by @dhruvmanila).
- Add more guidance to the ecosystem-minimization skill (and tweak the
script for setting up a primer project) so that agents are likely to
reproduce results in a deterministic and helpful way.

## Test Plan

I repeatedly ran `/review` with Codex on this until the only remaining
issues were trivial.