feat(hyperguild): brain HTTP REST client

Adds brainClient with Query and Write methods against the brain's
HTTP REST endpoints (/query, /write). Constructor reads BRAIN_URL env
var, defaulting to http://koala:30330 — the Tailscale-exposed
NodePort that serves both MCP and REST.

Tests cover success, transport error, and non-200 cases via
httptest fakes; URL override is verified via t.Setenv.

cmd/hyperguild/http_test.go

@@ -0,0 +1,88 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBrainClient_Query_Success(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		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"))
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"results":[{"path":"knowledge/x.md","title":"x","excerpt":"...","score":7}]}`))
+	}))
+	defer srv.Close()
+
+	c := &brainClient{baseURL: srv.URL, http: srv.Client()}
+	res, err := c.Query(context.Background(), "find-h", 3)
+	require.NoError(t, err)
+	require.Len(t, res.Results, 1)
+	assert.Equal(t, "knowledge/x.md", res.Results[0].Path)
+	assert.Equal(t, 7, res.Results[0].Score)
+}
+
+func TestBrainClient_Query_TransportError(t *testing.T) {
+	c := &brainClient{baseURL: "http://127.0.0.1:1", http: http.DefaultClient}
+	_, err := c.Query(context.Background(), "x", 5)
+	assert.Error(t, err)
+}
+
+func TestBrainClient_Query_Non200(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusInternalServerError)
+		_, _ = w.Write([]byte("boom"))
+	}))
+	defer srv.Close()
+
+	c := &brainClient{baseURL: srv.URL, http: srv.Client()}
+	_, err := c.Query(context.Background(), "x", 5)
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "500")
+}
+
+func TestBrainClient_Write_Success(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/write", r.URL.Path)
+		assert.Equal(t, http.MethodPost, r.Method)
+		body, _ := io.ReadAll(r.Body)
+		var got struct {
+			Type    string `json:"type"`
+			Slug    string `json:"slug"`
+			Content string `json:"content"`
+		}
+		require.NoError(t, json.Unmarshal(body, &got))
+		assert.Equal(t, "knowledge", got.Type)
+		assert.Equal(t, "find-h", got.Slug)
+		assert.Equal(t, "# body\n", got.Content)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"path":"knowledge/find-h.md"}`))
+	}))
+	defer srv.Close()
+
+	c := &brainClient{baseURL: srv.URL, http: srv.Client()}
+	res, err := c.Write(context.Background(), "knowledge", "find-h", strings.NewReader("# body\n"))
+	require.NoError(t, err)
+	assert.Equal(t, "knowledge/find-h.md", res.Path)
+}
+
+func TestNewBrainClient_DefaultURL(t *testing.T) {
+	t.Setenv("BRAIN_URL", "")
+	c := newBrainClient()
+	assert.Equal(t, "http://koala:30330", c.baseURL)
+}
+
+func TestNewBrainClient_OverrideURL(t *testing.T) {
+	t.Setenv("BRAIN_URL", "http://localhost:9999")
+	c := newBrainClient()
+	assert.Equal(t, "http://localhost:9999", c.baseURL)
+}