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/app/app.go

@@ -187,6 +187,7 @@ func routes(logger *slog.Logger, cfg *config.Config, info buildinfo.Info, db *st
 		Meals:         tools.NewMealsTool(db),
 		CRM:           tools.NewCRMTool(db),
 		Skills:        tools.NewSkillsTool(db, activeProjects),
+		ChatHistory:   tools.NewChatHistoryTool(db, activeProjects),
 	}
 
 	mcpHandler, err := mcpserver.New(cfg.MCP, logger, toolSet, activeProjects.Clear)