Improve TUI system prompt handling

+ allow setting a default agent

pkg/tui/model/model.go

@@ -37,6 +37,25 @@ func (m *AppModel) ClearConversation() {
 	m.RootMessages = []api.Message{}
 }
 
+func (m *AppModel) ApplySystemPrompt() {
+	var system string
+	agent := m.Ctx.GetAgent(m.Ctx.Config.Defaults.Agent)
+	if agent != nil && agent.SystemPrompt != "" {
+		system = agent.SystemPrompt
+	}
+	if system == "" {
+		system = m.Ctx.DefaultSystemPrompt()
+	}
+	if system != "" {
+		m.Messages = api.ApplySystemPrompt(m.Messages, system, false)
+	}
+}
+
+func (m *AppModel) NewConversation() {
+	m.ClearConversation()
+	m.ApplySystemPrompt()
+}
+
 func (m *AppModel) LoadConversations() (error, []LoadedConversation) {
 	messages, err := m.Ctx.Store.LatestConversationMessages()
 	if err != nil {