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) {

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 {

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}]}`))
 	}))