feat(ingestion): mount MCP handler at POST /mcp

The ingestion server now exposes both REST and MCP on the same port
(3300). MCP shares brainDir, pipeline config, and LLM client with the
REST handlers — single source of process state.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ingestion/internal/mcp/integration_test.go

@@ -0,0 +1,35 @@
+package mcp_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/mcp"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMCPMountedHandler(t *testing.T) {
+	srv := mcp.NewServer(t.TempDir(), nil, nil)
+	mux := http.NewServeMux()
+	mux.Handle("POST /mcp", srv)
+
+	ts := httptest.NewServer(mux)
+	defer ts.Close()
+
+	body, err := json.Marshal(map[string]any{
+		"jsonrpc": "2.0", "id": 1, "method": "tools/list",
+	})
+	require.NoError(t, err)
+	resp, err := http.Post(ts.URL+"/mcp", "application/json", bytes.NewReader(body))
+	require.NoError(t, err)
+	defer resp.Body.Close()
+	assert.Equal(t, http.StatusOK, resp.StatusCode)
+
+	out, _ := io.ReadAll(resp.Body)
+	assert.Contains(t, string(out), `"brain_query"`)
+}