Migrate To Shoehorn

This automates the migration from `as` type assertions to shoehorn in your test files. It's specifically for cleaning up tests where you're faking large objects but only care about a few properties, or intentionally passing bad data to test error paths. The skill installs the shoehorn package, searches for `as` assertions in test files, and replaces them with `fromPartial()` or `fromAny()` depending on whether you're passing partial valid data or deliberately wrong types. It's a focused refactor that makes test setup less brittle without changing runtime behavior. Only applies to test code, which is the right call since shoehorn should never touch production.