test(env): fix CI test failures for getCI() on Ubuntu and Windows

On CI systems (GitHub Actions Ubuntu/Windows), the CI environment variable
exists in process.env. After clearEnv('CI'), getCI() was falling back to
process.env.CI which is truthy on CI systems, causing tests to fail.

Fixed by using vi.stubEnv to clear the actual CI environment variable
before testing the cleared state.

Author: jdalton

test/unit/env/rewire.test.mts

@@ -56,9 +56,12 @@ describe('env rewiring', () => {
       setEnv('CI', '')
       expect(getCI()).toBe(true)
 
-      // CI returns false only when cleared
+      // CI returns false only when cleared AND not in process.env
+      // On CI systems, process.env.CI exists, so stub it out first
+      vi.stubEnv('CI', undefined)
       clearEnv('CI')
       expect(getCI()).toBe(false)
+      vi.unstubAllEnvs()
     })
 
     it('should allow undefined overrides', () => {
@@ -100,8 +103,11 @@ describe('env rewiring', () => {
       setEnv('CI', 'false')
       expect(getCI()).toBe(true) // Key exists, so true
 
+      // Clear both override and process.env (for CI systems where CI is set)
+      vi.stubEnv('CI', undefined)
       clearEnv('CI')
       expect(getCI()).toBe(false) // Key doesn't exist
+      vi.unstubAllEnvs()
     })
 
     it('test 3: should not be affected by previous tests', () => {