test: make unsafe test coercion explicit

Move test coercion out of a hidden global and require each test to import the helper so review tools and runtime scripts can see the unsafe boundary.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

test-setup.ts

@@ -12,8 +12,6 @@ const { restoreModuleMocks } = installModuleMockLifecycle(mock)
 let environmentSnapshot: NodeJS.ProcessEnv = { ...process.env }
 let workingDirectorySnapshot = process.cwd()
 
-globalThis.testCoerce = <TValue>(value: TValue): TValue => value
-
 function cleanupOmoCacheDir(cacheDir: string): void {
   rmSync(cacheDir, { recursive: true, force: true })
 }