Improve TUI system prompt handling

+ allow setting a default agent

pkg/tui/views/chat/chat.go

@@ -139,17 +139,6 @@ func Chat(app *model.AppModel) *Model {
 	m.replyCursor.SetChar(" ")
 	m.replyCursor.Focus()
 
-	system := app.Ctx.DefaultSystemPrompt()
-
-	agent := app.Ctx.GetAgent(app.Ctx.Config.Defaults.Agent)
-	if agent != nil && agent.SystemPrompt != "" {
-		system = agent.SystemPrompt
-	}
-
-	if system != "" {
-		m.App.Messages = api.ApplySystemPrompt(m.App.Messages, system, false)
-	}
-
 	m.input.Focus()
 	m.input.MaxHeight = 0
 	m.input.CharLimit = 0