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/brain_test.go

@@ -49,9 +49,15 @@ func TestRunBrainQuery_JSON(t *testing.T) {
 }
 
 func TestRunBrainQuery_Limit(t *testing.T) {
-	gotLimit := ""
+	gotLimit := -1
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		gotLimit = r.URL.Query().Get("limit")
+		body, _ := io.ReadAll(r.Body)
+		var p struct {
+			Query string `json:"query"`
+			Limit int    `json:"limit"`
+		}
+		_ = json.Unmarshal(body, &p)
+		gotLimit = p.Limit
 		_, _ = w.Write([]byte(`{"results":[]}`))
 	}))
 	defer srv.Close()
@@ -60,7 +66,7 @@ func TestRunBrainQuery_Limit(t *testing.T) {
 	var out, errBuf bytes.Buffer
 	err := runBrain(context.Background(), []string{"query", "--limit", "12", "topic"}, strings.NewReader(""), &out, &errBuf)
 	require.NoError(t, err)
-	assert.Equal(t, "12", gotLimit)
+	assert.Equal(t, 12, gotLimit)
 }
 
 func TestRunBrainQuery_MissingTopic(t *testing.T) {