fix nil pointer dereference on /agent command

NewDispatcher was not passed to AgentCoreConfig in the dbus session
manager, causing a nil function call when switching agents via /agent.

main.go

@@ -113,13 +113,14 @@ func (m *SessionManager) CreateSession(cwd, backendName, modelName, agentName st
 
 	// Create the agent core
 	core := agent.NewAgentCore(agent.AgentCoreConfig{
-		Backend:     be,
-		AgentName:   agentName,
-		AgentsDir:   agentsDir,
-		SessionsDir: sessionsDir,
-		SessionID:   sessID,
-		CWD:         cwd,
-		Runtime:     rt,
+		Backend:       be,
+		AgentName:     agentName,
+		AgentsDir:     agentsDir,
+		SessionsDir:   sessionsDir,
+		SessionID:     sessID,
+		CWD:           cwd,
+		Runtime:       rt,
+		NewDispatcher: newDisp,
 	})
 
 	ctx, cancel := context.WithCancel(context.Background())