feat: add chat history MCP tools

Adds save/get/list/delete tools for persisting and retrieving agent
chat histories in AMCS.

Changes:
- migrations/018_chat_histories.sql: new chat_histories table with
  indexes on session_id, project_id, channel, agent_id, created_at,
  and FTS over title+summary
- internal/types/extensions.go: ChatMessage and ChatHistory types
- internal/store/chat_histories.go: SaveChatHistory, GetChatHistory,
  GetChatHistoryBySessionID, ListChatHistories, DeleteChatHistory
- internal/tools/chat_history.go: ChatHistoryTool with four handlers
  (save_chat_history, get_chat_history, list_chat_histories,
  delete_chat_history)
- internal/mcpserver/server.go: ChatHistory field in ToolSet,
  registerChatHistoryTools registration function
- internal/app/app.go: wire ChatHistoryTool into ToolSet

internal/types/extensions.go

@@ -236,3 +236,24 @@ type AgentGuardrail struct {
 	CreatedAt   time.Time `json:"created_at"`
 	UpdatedAt   time.Time `json:"updated_at"`
 }
+
+// Chat Histories
+
+type ChatMessage struct {
+	Role    string `json:"role"`
+	Content string `json:"content"`
+}
+
+type ChatHistory struct {
+	ID        uuid.UUID      `json:"id"`
+	SessionID string         `json:"session_id"`
+	Title     string         `json:"title,omitempty"`
+	Channel   string         `json:"channel,omitempty"`
+	AgentID   string         `json:"agent_id,omitempty"`
+	ProjectID *uuid.UUID     `json:"project_id,omitempty"`
+	Messages  []ChatMessage  `json:"messages"`
+	Summary   string         `json:"summary,omitempty"`
+	Metadata  map[string]any `json:"metadata"`
+	CreatedAt time.Time      `json:"created_at"`
+	UpdatedAt time.Time      `json:"updated_at"`
+}