feat(hyperguild): brain pass-rate subcommand

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.

cmd/hyperguild/brain.go

@@ -11,15 +11,17 @@ import (
 
 func runBrain(ctx context.Context, args []string, stdin io.Reader, stdout, stderr io.Writer) error {
 	if len(args) == 0 {
-		return errors.New("subcommand required (query|write)")
+		return errors.New("subcommand required (query|write|pass-rate)")
 	}
 	switch args[0] {
 	case "query":
 		return runBrainQuery(ctx, args[1:], stdin, stdout, stderr)
 	case "write":
 		return runBrainWrite(ctx, args[1:], stdin, stdout, stderr)
+	case "pass-rate":
+		return runBrainPassRate(ctx, args[1:], stdin, stdout, stderr)
 	default:
-		return fmt.Errorf("unknown subcommand: %s (expected query|write)", args[0])
+		return fmt.Errorf("unknown subcommand: %s (expected query|write|pass-rate)", args[0])
 	}
 }
 
@@ -71,3 +73,34 @@ func runBrainWrite(ctx context.Context, args []string, stdin io.Reader, stdout,
 	fmt.Fprintln(stdout, res.Path) //nolint:errcheck
 	return nil
 }
+
+func runBrainPassRate(ctx context.Context, args []string, _ io.Reader, stdout, stderr io.Writer) error {
+	fs := flag.NewFlagSet("brain pass-rate", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+	asJSON := fs.Bool("json", false, "output JSON instead of human-readable")
+	window := fs.String("window", "7d", "lookback window (e.g. 1h, 24h, 7d, 30d)")
+	if err := fs.Parse(args); err != nil {
+		return fmt.Errorf("parse flags: %w", err)
+	}
+	if fs.NArg() < 1 {
+		return errors.New("skill required")
+	}
+	skill := fs.Arg(0)
+
+	res, err := newBrainClient().PassRate(ctx, skill, *window)
+	if err != nil {
+		return err
+	}
+
+	if *asJSON {
+		enc := json.NewEncoder(stdout)
+		enc.SetIndent("", "  ")
+		return enc.Encode(res)
+	}
+	if res.PassRate == nil {
+		fmt.Fprintf(stdout, "%s: no data (window: %s)\n", res.Skill, res.Window) //nolint:errcheck
+		return nil
+	}
+	fmt.Fprintf(stdout, "%s: %d / %d = %.0f%% (window: %s)\n", res.Skill, res.Pass, res.Total, *res.PassRate*100, res.Window) //nolint:errcheck
+	return nil
+}