refactor(testgen): enforce stricter rules in Go test prompt

Update the test generation prompt for Go to prohibit monkey-patching, global variable reassignments, and reflect tricks. Mandate use of only idiomatic Go with real function calls, table-driven tests, and simple happy/error path coverage. Simplify unit and live test structures for better production readiness.

cmd/testgen.go

@@ -206,26 +206,29 @@ func getTestPrompt(lang string) string {
 	case "Go":
 		return `You are an expert Go test writer.
 
-Generate a COMPLETE, production-ready *_test.go file that follows this modern Unit + Live test pattern:
+Generate a COMPLETE, production-ready *_test.go file using ONLY idiomatic Go.
 
-1. Fast unit test (always runs on "go test"):
+STRICT RULES — NEVER VIOLATE THESE:
+- NO monkey-patching: never assign to runtime.GOOS, exec.Command, os.Stdout, os.Exit, or ANY package-level func/var
+- NO global variable reassignment
+- NO reflect tricks for mocking
+- Use ONLY real function calls + table-driven tests
+- For os.Exit or stdout testing, use simple smoke tests or pipes (no reassignment)
+- Prefer simple happy-path + error-path tests
+
+1. Fast unit test:
    func TestXXX_Unit(t *testing.T) {
        t.Parallel()
        t.Log("✓ Fast XXX unit test")
-
-       // Use table-driven tests with real function calls
-       // Cover happy path + error cases
-       // NO monkey-patching: never assign to runtime.GOOS, exec.Command, os/exec.Command, or any exported var/func
-       // Use only standard testing.T (no testify unless the project already imports it)
+       // table-driven, real calls only
    }
 
-2. Optional live integration test (skipped by default):
+2. Optional live test:
    func TestXXX_Live(t *testing.T) {
        if !testing.Short() {
            t.Skip("skipping live integration test. Run with:\n      go test -run TestXXX_Live -short -v")
        }
        t.Log("🧪 Running live integration test...")
-       // Real calls only
    }
 
 Exact rules: