feat: implement dynamic context compression for efficient memory usage

- Added Summary field to Session struct
- Implemented background summarization when history > 20 messages
- Included conversation summary in system prompt for long-term context
- Added thread-safety for concurrent summarization per session

pkg/session/manager.go

@@ -13,6 +13,7 @@ import (
 type Session struct {
 	Key      string              `json:"key"`
 	Messages []providers.Message `json:"messages"`
+	Summary  string              `json:"summary,omitempty"`
 	Created  time.Time           `json:"created"`
 	Updated  time.Time           `json:"updated"`
 }
@@ -92,6 +93,45 @@ func (sm *SessionManager) GetHistory(key string) []providers.Message {
 	return history
 }
 
+func (sm *SessionManager) GetSummary(key string) string {
+	sm.mu.RLock()
+	defer sm.mu.RUnlock()
+
+	session, ok := sm.sessions[key]
+	if !ok {
+		return ""
+	}
+	return session.Summary
+}
+
+func (sm *SessionManager) SetSummary(key string, summary string) {
+	sm.mu.Lock()
+	defer sm.mu.Unlock()
+
+	session, ok := sm.sessions[key]
+	if ok {
+		session.Summary = summary
+		session.Updated = time.Now()
+	}
+}
+
+func (sm *SessionManager) TruncateHistory(key string, keepLast int) {
+	sm.mu.Lock()
+	defer sm.mu.Unlock()
+
+	session, ok := sm.sessions[key]
+	if !ok {
+		return
+	}
+
+	if len(session.Messages) <= keepLast {
+		return
+	}
+
+	session.Messages = session.Messages[len(session.Messages)-keepLast:]
+	session.Updated = time.Now()
+}
+
 func (sm *SessionManager) Save(session *Session) error {
 	if sm.storage == "" {
 		return nil