Adds 'hyperguild brain pass-rate <skill> [--window 7d] [--json]' calling the new /pass-rate endpoint. Human output: tdd: 47 / 50 = 94% (window: 7d) or 'no data (window: 7d)' when pass_rate is null. PassRateResult mirrors the response envelope; PassRate is *float64 so null is preserved across decode.