Update system prompt handling (again)

Add `api.ApplySystemPrompt`, renamed `GetSystemPrompt` to
`DefaultSystemPrompt`.

pkg/api/message.go

@@ -31,6 +31,20 @@ type Message struct {
 	SelectedReply   *Message `gorm:"foreignKey:SelectedReplyID"`
 }
 
+func ApplySystemPrompt(m []Message, system string, force bool) []Message {
+	if len(m) > 0 && m[0].Role == MessageRoleSystem {
+		if force {
+			m[0].Content = system
+		}
+		return m
+	} else {
+		return append([]Message{{
+			Role:    MessageRoleSystem,
+			Content: system,
+		}}, m...)
+	}
+}
+
 func (m *MessageRole) IsAssistant() bool {
 	switch *m {
 	case MessageRoleAssistant, MessageRoleToolCall: