feat: send workspace images to QQ chats

Worker results may report image attachments stored inside the workspace;
the runtime validates them (containment, png/jpg magic, size) and
delivers them with the pending event. Assistants gain a send_image
action so users can ask for an image later. QQ uploads via /files with
base64 file_data and sends msg_type 7 rich media, sharing the same
msg_id/msg_seq counter as text replies; non-image adapters flatten
images to text.

src/core/proposal-store.ts

@@ -19,11 +19,18 @@ export type ProposalStatus =
   | "pending"
   | "finished";
 
+export interface ProposalPendingAttachment {
+  path: string;
+  mimeType?: string;
+}
+
 export interface ProposalPending {
   summary: string;
   question?: string;
   workspaceDirty?: boolean;
   receivedAt: number;
+  attachments?: ProposalPendingAttachment[];
+  droppedAttachments?: string[];
 }
 
 export type ProposalFinishKind = "done" | "cancelled";
@@ -325,6 +332,17 @@ function validateProposal(proposal: Proposal, id: string): void {
       || typeof pending.receivedAt !== "number") {
       invalid("pending");
     }
+    if (pending.attachments !== undefined
+      && (!Array.isArray(pending.attachments) || pending.attachments.length > 3
+        || pending.attachments.some((attachment) => !isRecord(attachment)
+          || typeof attachment.path !== "string" || attachment.path.length === 0
+          || (attachment.mimeType !== undefined && typeof attachment.mimeType !== "string")))) {
+      invalid("pending.attachments");
+    }
+    if (pending.droppedAttachments !== undefined
+      && (!Array.isArray(pending.droppedAttachments) || pending.droppedAttachments.some((entry) => typeof entry !== "string"))) {
+      invalid("pending.droppedAttachments");
+    }
   }
   if (proposal.finishKind !== undefined && !FINISH_KINDS.includes(proposal.finishKind)) invalid("finishKind");
   if (proposal.finishNote !== undefined && typeof proposal.finishNote !== "string") invalid("finishNote");