Add testing for examples

[fogfish]

Problem: Examples might break but we won't know until someone runs them.

Required Changes:

1. Add examples/example_test.go:

func TestExamplesCompile(t *testing.T) {
    // Test that all examples compile
    examples := []string{
        "01_helloworld",
        "02_rainbow",
        // ...
    }
    
    for _, ex := range examples {
        cmd := exec.Command("go", "build", "./"+ex)
        if err := cmd.Run(); err != nil {
            t.Errorf("Example %s failed to compile: %v", ex, err)
        }
    }
}

2. Add to CI pipeline
3. Consider making examples buildable without LLM credentials

Estimated Effort: 1 hour
Skills Required: Testing, CI/CD