feat: add Timestamp field to Message struct

- Add Timestamp field to Message struct in pkg/providers/types.go
- Automatically set timestamp in AddMessage and AddFullMessage methods in pkg/session/manager.go
- Timestamp is stored as Unix timestamp (int64)

pkg/providers/types.go

@@ -33,6 +33,7 @@ type Message struct {
 	Content    string     `json:"content"`
 	ToolCalls  []ToolCall `json:"tool_calls,omitempty"`
 	ToolCallID string     `json:"tool_call_id,omitempty"`
+	Timestamp  int64      `json:"timestamp,omitempty"`
 }
 
 type LLMProvider interface {

pkg/session/manager.go

@@ -63,6 +63,7 @@ func (sm *SessionManager) AddMessage(sessionKey, role, content string) {
 	sm.AddFullMessage(sessionKey, providers.Message{
 		Role:    role,
 		Content: content,
+		Timestamp: time.Now().Unix(),
 	})
 }
 
@@ -82,6 +83,11 @@ func (sm *SessionManager) AddFullMessage(sessionKey string, msg providers.Messag
 		sm.sessions[sessionKey] = session
 	}
 
+	// Set timestamp if not already set
+	if msg.Timestamp == 0 {
+		msg.Timestamp = time.Now().Unix()
+	}
+
 	session.Messages = append(session.Messages, msg)
 	session.Updated = time.Now()
 }