fix(mcp): harden subsystem interface contracts

pkg/mcp/agentic/prep.go

@@ -36,6 +36,12 @@ type PrepSubsystem struct {
 	notifier   coremcp.Notifier
 }
 
+var (
+	_ coremcp.Subsystem             = (*PrepSubsystem)(nil)
+	_ coremcp.SubsystemWithShutdown = (*PrepSubsystem)(nil)
+	_ coremcp.SubsystemWithNotifier = (*PrepSubsystem)(nil)
+)
+
 // NewPrep creates an agentic subsystem.
 //
 //	prep := NewPrep()

pkg/mcp/brain/brain.go

@@ -23,6 +23,12 @@ type Subsystem struct {
 	notifier coremcp.Notifier
 }
 
+var (
+	_ coremcp.Subsystem             = (*Subsystem)(nil)
+	_ coremcp.SubsystemWithShutdown = (*Subsystem)(nil)
+	_ coremcp.SubsystemWithNotifier = (*Subsystem)(nil)
+)
+
 // New creates a brain subsystem that uses the given IDE bridge for Laravel communication.
 //
 //	brain := New(ideBridge)

pkg/mcp/brain/direct.go

@@ -35,6 +35,12 @@ type DirectSubsystem struct {
 	onChannel channelSender
 }
 
+var (
+	_ coremcp.Subsystem                    = (*DirectSubsystem)(nil)
+	_ coremcp.SubsystemWithShutdown        = (*DirectSubsystem)(nil)
+	_ coremcp.SubsystemWithChannelCallback = (*DirectSubsystem)(nil)
+)
+
 // OnChannel sets a callback for channel event broadcasting.
 // Called by the MCP service after creation to wire up notifications.
 //

pkg/mcp/ide/ide.go

@@ -35,6 +35,12 @@ type Subsystem struct {
 	activity     []ActivityEvent
 }
 
+var (
+	_ coremcp.Subsystem             = (*Subsystem)(nil)
+	_ coremcp.SubsystemWithShutdown = (*Subsystem)(nil)
+	_ coremcp.SubsystemWithNotifier = (*Subsystem)(nil)
+)
+
 // New creates an IDE subsystem from a Config DTO.
 //
 //	cfg := DefaultConfig()