feat(mcp): add ChannelPush IPC message + HandleIPCEvents

Services can now push channel events to Claude Code by sending a
ChannelPush message via Core IPC. The MCP service catches it in
HandleIPCEvents and calls ChannelSend to the stdio transport.

- ChannelPush{Channel, Data} message type in subsystem.go
- HandleIPCEvents on Service catches ChannelPush → ChannelSend
- Enables runner→mcp→Claude Code notification pipeline

Co-Authored-By: Virgil <virgil@lethean.io>

pkg/mcp/register.go

@@ -80,6 +80,18 @@ func (s *Service) OnStartup(ctx context.Context) core.Result {
 	return core.Result{OK: true}
 }
 
+// HandleIPCEvents implements Core's IPC handler interface.
+// Catches ChannelPush messages from other services and pushes them to Claude Code sessions.
+//
+//	c.ACTION(mcp.ChannelPush{Channel: "agent.status", Data: statusMap})
+func (s *Service) HandleIPCEvents(c *core.Core, msg core.Message) core.Result {
+	switch ev := msg.(type) {
+	case ChannelPush:
+		s.ChannelSend(context.Background(), ev.Channel, ev.Data)
+	}
+	return core.Result{OK: true}
+}
+
 // OnShutdown implements core.Stoppable — stops the MCP transport.
 func (s *Service) OnShutdown(ctx context.Context) core.Result {
 	if err := s.Shutdown(ctx); err != nil {

pkg/mcp/subsystem.go

@@ -38,8 +38,15 @@ type Notifier interface {
 	ChannelSend(ctx context.Context, channel string, data any)
 }
 
-// Compile-time assertion: *Service implements Notifier.
-var _ Notifier = (*Service)(nil)
+// ChannelPush is a Core IPC message that any service can send to push
+// a channel event to connected Claude Code sessions.
+// The MCP service catches this in HandleIPCEvents and calls ChannelSend.
+//
+//	c.ACTION(mcp.ChannelPush{Channel: "agent.status", Data: map[string]any{"repo": "go-io"}})
+type ChannelPush struct {
+	Channel string
+	Data    any
+}
 
 // SubsystemWithNotifier extends Subsystem for those that emit channel events.
 // SetNotifier is called after New() before any tool calls.