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
