fix(acp): address review feedback on media attachment forwarding

- runtime.ts: skip empty text block when input.text is empty; only emit
  type: "image" blocks for image/* MIME types — non-image attachments
  (documents, PDFs, audio, video) are silently skipped as the ACP protocol
  does not define a binary content block for them
- dispatch-acp.ts: add 10 MB file size guard in resolveAcpAttachments to
  prevent unbounded memory allocation when large files are inbound

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

extensions/acpx/src/runtime.ts

@@ -311,9 +311,16 @@ export class AcpxRuntime implements AcpRuntime {
     });
 
     if (input.attachments && input.attachments.length > 0) {
-      const blocks: unknown[] = [{ type: "text", text: input.text }];
+      const blocks: unknown[] = [];
+      if (input.text) {
+        blocks.push({ type: "text", text: input.text });
+      }
       for (const attachment of input.attachments) {
-        blocks.push({ type: "image", mimeType: attachment.mediaType, data: attachment.data });
+        if (attachment.mediaType.startsWith("image/")) {
+          blocks.push({ type: "image", mimeType: attachment.mediaType, data: attachment.data });
+        }
+        // Non-image attachments (documents, PDFs, audio, video) are not supported
+        // as binary content blocks in the ACP protocol — skip silently.
       }
       child.stdin.end(JSON.stringify(blocks));
     } else {

src/auto-reply/reply/dispatch-acp.ts

@@ -63,6 +63,8 @@ function resolveAcpPromptText(ctx: FinalizedMsgContext): string {
   ]).trim();
 }
 
+const ACP_ATTACHMENT_MAX_BYTES = 10 * 1024 * 1024; // 10 MB — consistent with Telegram mediaMaxMb default
+
 async function resolveAcpAttachments(ctx: FinalizedMsgContext): Promise<AcpTurnAttachment[]> {
   const mediaAttachments = normalizeAttachments(ctx);
   const results: AcpTurnAttachment[] = [];
@@ -72,6 +74,13 @@ async function resolveAcpAttachments(ctx: FinalizedMsgContext): Promise<AcpTurnA
       continue;
     }
     try {
+      const stat = await fs.stat(filePath);
+      if (stat.size > ACP_ATTACHMENT_MAX_BYTES) {
+        logVerbose(
+          `dispatch-acp: skipping attachment ${filePath} (${stat.size} bytes exceeds ${ACP_ATTACHMENT_MAX_BYTES} byte limit)`,
+        );
+        continue;
+      }
       const buf = await fs.readFile(filePath);
       results.push({ mediaType: attachment.mime ?? "application/octet-stream", data: buf.toString("base64") });
     } catch {