feat: admit photos and image documents into the bound session

Telegram 图片消息此前一律回复「暂只支持文本消息」。本改动：

- photo 取最大尺寸（TG 压成 jpeg），document 仅接受 image/png|jpeg|webp|gif
- caption 作为伴随文本块进入同一条消息（不解析为命令）
- 相册（同一 media_group_id）去抖 1.2s 合并为一条多图消息，避免 N 连回合
- 派发走宿主 sessions.prompt RPC（Web 端同通道）：模型图像能力校验 +
  附件服务持久化 + mode queue 排队，与纯文本路径的 agent.followup 语义一致
- 错误映射：MODEL_DOES_NOT_SUPPORT_IMAGES → 提示换多模态模型；其余回退
  图片发送失败 + [code] 详情；下载失败即时提示
- 客户端新增 getFile/downloadFile（GET file 端点，复用代理感知 fetch）

tests +7（58/58）：photo+caption、png document、pdf 拒绝、相册合并、
模型不支持、下载失败、未绑定。无 emoji 文案。

lib/client.d.ts

@@ -10,12 +10,38 @@ export interface TelegramChat {
     title?: string;
     username?: string;
 }
+export interface TelegramPhotoSize {
+    file_id: string;
+    file_unique_id: string;
+    width: number;
+    height: number;
+    file_size?: number;
+}
+export interface TelegramDocument {
+    file_id: string;
+    file_unique_id: string;
+    file_name?: string;
+    mime_type?: string;
+    file_size?: number;
+}
 export interface TelegramMessage {
     message_id: number;
     date: number;
     chat: TelegramChat;
     from?: TelegramUser;
     text?: string;
+    /** Media caption (photos/documents); TG puts it here, not in `text`. */
+    caption?: string;
+    photo?: TelegramPhotoSize[];
+    document?: TelegramDocument;
+    /** Album grouping id: several photo updates share one media_group_id. */
+    media_group_id?: string;
+}
+export interface TelegramFile {
+    file_id: string;
+    file_unique_id: string;
+    file_size?: number;
+    file_path?: string;
 }
 export interface TelegramCallbackQuery {
     id: string;
@@ -56,6 +82,10 @@ export interface TelegramClientLike {
     sendChatAction(chatId: number, action: string): Promise<boolean>;
     answerCallbackQuery(callbackQueryId: string, text?: string): Promise<boolean>;
     setMyCommands(commands: TelegramBotCommand[]): Promise<boolean>;
+    /** Resolve a file_id to a file_path (downloadable URL suffix). */
+    getFile(fileId: string): Promise<TelegramFile>;
+    /** Download file bytes from the Bot API file endpoint (GET, proxy-aware). */
+    downloadFile(filePath: string): Promise<Uint8Array>;
 }
 export declare class TelegramClient implements TelegramClientLike {
     private readonly token;
@@ -72,4 +102,6 @@ export declare class TelegramClient implements TelegramClientLike {
     sendChatAction(chatId: number, action: string): Promise<boolean>;
     answerCallbackQuery(callbackQueryId: string, text?: string): Promise<boolean>;
     setMyCommands(commands: TelegramBotCommand[]): Promise<boolean>;
+    getFile(fileId: string): Promise<TelegramFile>;
+    downloadFile(filePath: string): Promise<Uint8Array>;
 }