fix(mcp): do not respond to JSON-RPC notifications

The supervisor's MCP HTTP handler was answering every parsed request,
including notifications (messages with no id field). Per JSON-RPC 2.0,
notifications must not receive a response. The Apr-29 incident saw
Claude Code's MCP client receive a -32601 error for the standard
notifications/initialized handshake step and disconnect immediately
after a successful initialize.

Skip writing the response when req.ID == nil. Cover both the
known-method (notifications/initialized) and unknown-method paths
with tests.

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

internal/mcp/server_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"net/http"
 	"net/http/httptest"
+	"strings"
 	"testing"
 
 	"github.com/mathiasbq/supervisor/internal/mcp"
@@ -76,3 +77,39 @@ func TestMCPUnknownMethod(t *testing.T) {
 	require.NoError(t, json.Unmarshal(rr.Body.Bytes(), &resp))
 	assert.NotNil(t, resp["error"])
 }
+
+func TestMCPNotificationKnownMethodGetsNoResponseBody(t *testing.T) {
+	reg := registry.New()
+	srv := mcp.NewServer(reg)
+
+	// JSON-RPC 2.0 notification: "id" field absent. Per spec, server MUST NOT
+	// reply. notifications/initialized is part of the standard MCP handshake.
+	req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
+		"jsonrpc": "2.0",
+		"method":  "notifications/initialized",
+	}))
+	req.Header.Set("Content-Type", "application/json")
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, req)
+
+	assert.Equal(t, http.StatusOK, rr.Code)
+	assert.Empty(t, strings.TrimSpace(rr.Body.String()),
+		"notifications must not receive a response body")
+}
+
+func TestMCPNotificationUnknownMethodGetsNoResponseBody(t *testing.T) {
+	reg := registry.New()
+	srv := mcp.NewServer(reg)
+
+	req := httptest.NewRequest(http.MethodPost, "/mcp", jsonBody(t, map[string]any{
+		"jsonrpc": "2.0",
+		"method":  "notifications/totally-unknown",
+	}))
+	req.Header.Set("Content-Type", "application/json")
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, req)
+
+	assert.Equal(t, http.StatusOK, rr.Code)
+	assert.Empty(t, strings.TrimSpace(rr.Body.String()),
+		"unknown notifications must also receive no response body")
+}