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。

src/client.ts

@@ -14,12 +14,41 @@ export interface TelegramChat {
   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 {
@@ -73,6 +102,10 @@ 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>
 }
 
 function resolveProxyUrl(): string | undefined {
@@ -197,4 +230,22 @@ export class TelegramClient implements TelegramClientLike {
   async setMyCommands(commands: TelegramBotCommand[]): Promise<boolean> {
     return this.call<boolean>('setMyCommands', { commands })
   }
+
+  async getFile(fileId: string): Promise<TelegramFile> {
+    return this.call<TelegramFile>('getFile', { file_id: fileId })
+  }
+
+  async downloadFile(filePath: string): Promise<Uint8Array> {
+    const url = `${this.baseUrl}/file/bot${this.token}/${filePath}`
+    try {
+      const response = await this.fetchImpl(url)
+      if (!response.ok) {
+        throw new Error(`Telegram file download failed: HTTP ${String(response.status)}`)
+      }
+      return new Uint8Array(await response.arrayBuffer())
+    } catch (err) {
+      const message = err instanceof Error ? err.message : String(err)
+      throw new Error(this.redact(message))
+    }
+  }
 }