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_test.go

@@ -15,9 +15,18 @@ import (
 
 func TestBrainClient_Query_Success(t *testing.T) {
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, http.MethodPost, r.Method)
 		assert.Equal(t, "/query", r.URL.Path)
-		assert.Equal(t, "find-h", r.URL.Query().Get("q"))
-		assert.Equal(t, "3", r.URL.Query().Get("limit"))
+
+		body, _ := io.ReadAll(r.Body)
+		var got struct {
+			Query string `json:"query"`
+			Limit int    `json:"limit"`
+		}
+		require.NoError(t, json.Unmarshal(body, &got))
+		assert.Equal(t, "find-h", got.Query)
+		assert.Equal(t, 3, got.Limit)
+
 		w.Header().Set("Content-Type", "application/json")
 		_, _ = w.Write([]byte(`{"results":[{"path":"knowledge/x.md","title":"x","excerpt":"...","score":7}]}`))
 	}))