feat: add multi-turn conversation memory to Agent

- Add chatHistory store (in-memory, per user, last 20 messages)
- Agent now passes full conversation context to LLM via CallWithRequest
- Records both user messages and assistant responses
- Add /clear command to reset conversation history
- Add 'Clear' quick action button in frontend
- Supports natural multi-turn conversations (follow-up questions, context)

agent/history.go

@@ -0,0 +1,86 @@
+package agent
+
+import (
+	"sync"
+	"time"
+)
+
+// chatMessage represents a single message in conversation history.
+type chatMessage struct {
+	Role      string    `json:"role"` // "user" or "assistant"
+	Content   string    `json:"content"`
+	Timestamp time.Time `json:"timestamp"`
+}
+
+// chatHistory stores conversation history per user.
+type chatHistory struct {
+	mu       sync.RWMutex
+	sessions map[int64][]chatMessage
+	maxTurns int // max messages per user (user+assistant pairs)
+}
+
+func newChatHistory(maxTurns int) *chatHistory {
+	if maxTurns <= 0 {
+		maxTurns = 20 // default: keep last 20 messages (10 turns)
+	}
+	return &chatHistory{
+		sessions: make(map[int64][]chatMessage),
+		maxTurns: maxTurns,
+	}
+}
+
+// Add appends a message to the user's history.
+func (h *chatHistory) Add(userID int64, role, content string) {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	h.sessions[userID] = append(h.sessions[userID], chatMessage{
+		Role:      role,
+		Content:   content,
+		Timestamp: time.Now(),
+	})
+
+	// Trim to maxTurns
+	msgs := h.sessions[userID]
+	if len(msgs) > h.maxTurns {
+		h.sessions[userID] = msgs[len(msgs)-h.maxTurns:]
+	}
+}
+
+// Get returns the conversation history for a user.
+func (h *chatHistory) Get(userID int64) []chatMessage {
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+
+	msgs := h.sessions[userID]
+	if msgs == nil {
+		return nil
+	}
+	// Return a copy
+	result := make([]chatMessage, len(msgs))
+	copy(result, msgs)
+	return result
+}
+
+// Clear resets conversation history for a user.
+func (h *chatHistory) Clear(userID int64) {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+	delete(h.sessions, userID)
+}
+
+// CleanOld removes sessions older than the given duration.
+func (h *chatHistory) CleanOld(maxAge time.Duration) {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	now := time.Now()
+	for uid, msgs := range h.sessions {
+		if len(msgs) > 0 {
+			lastMsg := msgs[len(msgs)-1]
+			if now.Sub(lastMsg.Timestamp) > maxAge {
+				delete(h.sessions, uid)
+			}
+		}
+	}
+}