[ty] Add `rule` subcommand
Summary
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Fixes astral-sh/ty#2061. I really just wanted a list of rules for my
categorization work, but I found the issue, and Claude knocked it out quickly.
No pressure to land (or review) this if there's more to it, I see that the issue
is labeled `wish`.
Test Plan
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New CLI tests mirroring the ones for Ruff
