feat: Add Telegram AI Assistant (moltbot-nofx integration)

- Add assistant package with AI Agent runtime
  - agent.go: Core agent loop with tool calling
  - session.go: Conversation memory management
  - tool.go: Tool interface and base implementation
  - trading_tools.go: Trading-specific tools (13 tools)
  - prompts.go: Trading expert system prompts (EN/ZH)

- Add telegram package for Telegram bot integration
  - bot.go: Telegram bot with rate limiting & access control
  - config.go: Environment-based configuration

- Update main.go to initialize Telegram bot on startup
- Update .env.example with new configuration options
- Add gopkg.in/telebot.v3 dependency

Trading tools available:
- Query: get_balance, get_positions, list_traders, get_trader_status
- Control: start_trader, stop_trader
- Trading: get_market_price, open_long, open_short, close_position
- Config: list_strategies, list_exchanges, list_ai_models

assistant/session.go

@@ -0,0 +1,122 @@
+package assistant
+
+import (
+	"sync"
+	"time"
+)
+
+// Message represents a single message in conversation
+type Message struct {
+	Role      string    `json:"role"` // "user", "assistant", "system", "tool"
+	Content   string    `json:"content"`
+	Timestamp time.Time `json:"timestamp"`
+	
+	// For tool messages
+	ToolName   string      `json:"tool_name,omitempty"`
+	ToolResult interface{} `json:"tool_result,omitempty"`
+}
+
+// Session represents a conversation session with memory
+type Session struct {
+	ID        string    `json:"id"`
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+	
+	// User info
+	UserID   string `json:"user_id"`
+	UserName string `json:"user_name"`
+	Platform string `json:"platform"` // "telegram", "web", etc.
+	
+	// Conversation history
+	messages    []Message
+	maxMessages int
+	mu          sync.RWMutex
+	
+	// Custom metadata
+	Metadata map[string]interface{} `json:"metadata"`
+}
+
+// NewSession creates a new conversation session
+func NewSession(id string, maxMessages int) *Session {
+	return &Session{
+		ID:          id,
+		CreatedAt:   time.Now(),
+		UpdatedAt:   time.Now(),
+		messages:    make([]Message, 0),
+		maxMessages: maxMessages,
+		Metadata:    make(map[string]interface{}),
+	}
+}
+
+// AddMessage adds a message to the session
+func (s *Session) AddMessage(msg Message) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.messages = append(s.messages, msg)
+	s.UpdatedAt = time.Now()
+
+	// Trim old messages if exceeding max
+	if len(s.messages) > s.maxMessages {
+		// Keep the most recent messages
+		s.messages = s.messages[len(s.messages)-s.maxMessages:]
+	}
+}
+
+// GetMessages returns a copy of all messages
+func (s *Session) GetMessages() []Message {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	result := make([]Message, len(s.messages))
+	copy(result, s.messages)
+	return result
+}
+
+// GetRecentMessages returns the N most recent messages
+func (s *Session) GetRecentMessages(n int) []Message {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	if n >= len(s.messages) {
+		result := make([]Message, len(s.messages))
+		copy(result, s.messages)
+		return result
+	}
+
+	result := make([]Message, n)
+	copy(result, s.messages[len(s.messages)-n:])
+	return result
+}
+
+// Clear removes all messages from the session
+func (s *Session) Clear() {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.messages = make([]Message, 0)
+	s.UpdatedAt = time.Now()
+}
+
+// SetUserInfo sets user information
+func (s *Session) SetUserInfo(userID, userName, platform string) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.UserID = userID
+	s.UserName = userName
+	s.Platform = platform
+}
+
+// SetMetadata sets a metadata value
+func (s *Session) SetMetadata(key string, value interface{}) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.Metadata[key] = value
+}
+
+// GetMetadata gets a metadata value
+func (s *Session) GetMetadata(key string) (interface{}, bool) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	v, ok := s.Metadata[key]
+	return v, ok
+}