fix(hyperguild): brain Query uses POST /query with JSON body

The brain HTTP REST /query endpoint accepts POST with JSON
{query, limit}, not GET with URL query string. Surfaced by
Task 7 smoke testing — GET returned 405 Method Not Allowed.

The response shape ({results:[...]}) is unchanged; only the
request side flips to POST + JSON body. brainClient.Write was
already using POST + JSON body and is unaffected.

Tests updated to assert POST + JSON body on the Query path.

cmd/hyperguild/http.go

@@ -7,9 +7,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
-	"net/url"
 	"os"
-	"strconv"
 	"time"
 )
 
@@ -49,23 +47,29 @@ type QueryResult struct {
 }
 
 func (c *brainClient) Query(ctx context.Context, topic string, limit int) (*QueryResult, error) {
-	q := url.Values{}
-	q.Set("q", topic)
-	q.Set("limit", strconv.Itoa(limit))
-	u := c.baseURL + "/query?" + q.Encode()
+	payload, err := json.Marshal(struct {
+		Query string `json:"query"`
+		Limit int    `json:"limit"`
+	}{Query: topic, Limit: limit})
+	if err != nil {
+		return nil, fmt.Errorf("marshal payload: %w", err)
+	}
 
-	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
+	u := c.baseURL + "/query"
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, u, bytes.NewReader(payload))
 	if err != nil {
 		return nil, fmt.Errorf("build request: %w", err)
 	}
+	req.Header.Set("Content-Type", "application/json")
+
 	resp, err := c.http.Do(req)
 	if err != nil {
-		return nil, fmt.Errorf("brain GET /query: %w", err)
+		return nil, fmt.Errorf("brain POST /query: %w", err)
 	}
 	defer func() { _ = resp.Body.Close() }()
 	if resp.StatusCode != http.StatusOK {
 		body, _ := io.ReadAll(resp.Body)
-		return nil, fmt.Errorf("brain GET /query: status %d: %s", resp.StatusCode, string(body))
+		return nil, fmt.Errorf("brain POST /query: status %d: %s", resp.StatusCode, string(body))
 	}
 	var out QueryResult
 	if err := json.NewDecoder(resp.Body).Decode(&out); err != nil {