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/http.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"net/url"
 	"os"
 	"time"
 )
@@ -119,3 +120,40 @@ func (c *brainClient) Write(ctx context.Context, kind, slug string, content io.R
 	}
 	return &out, nil
 }
+
+// PassRateResult mirrors the /pass-rate response envelope.
+type PassRateResult struct {
+	Skill    string   `json:"skill"`
+	Window   string   `json:"window"`
+	Pass     int      `json:"pass"`
+	Fail     int      `json:"fail"`
+	Skip     int      `json:"skip"`
+	Total    int      `json:"total"`
+	PassRate *float64 `json:"pass_rate"`
+}
+
+func (c *brainClient) PassRate(ctx context.Context, skill, window string) (*PassRateResult, error) {
+	q := url.Values{}
+	q.Set("skill", skill)
+	q.Set("window", window)
+	u := c.baseURL + "/pass-rate?" + q.Encode()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
+	if err != nil {
+		return nil, fmt.Errorf("build request: %w", err)
+	}
+	resp, err := c.http.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("brain GET /pass-rate: %w", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+	if resp.StatusCode != http.StatusOK {
+		body, _ := io.ReadAll(resp.Body)
+		return nil, fmt.Errorf("brain GET /pass-rate: status %d: %s", resp.StatusCode, string(body))
+	}
+	var out PassRateResult
+	if err := json.NewDecoder(resp.Body).Decode(&out); err != nil {
+		return nil, fmt.Errorf("decode /pass-rate response: %w", err)
+	}
+	return &out, nil
+}