feat(box): bidirectional attachment transfer for sandbox

Materialize inbound attachments into the sandbox workspace so agents can
process user-sent files, and collect agent-produced files from the outbox
to attach them back to the reply.

- box(service): add materialize_inbound_attachments / collect_outbound
  attachments. Prefer direct host-filesystem read/write on the bind-mounted
  workspace (no size limit), falling back to chunked exec only for
  non-shared backends (e2b/remote). Clear per-query inbox/outbox dirs at
  turn start to avoid query_id-reuse collisions.
- provider(localagent): inject inbound attachment descriptors into the
  sandbox and append a system note telling the agent the inbox/outbox paths.
- pipeline(wrapper): collect outbox files on the final stream chunk and
  append them as attachment components to the response chain.
- web(debug-dialog): render File components with a download link when
  base64/url is present; add base64/path fields to the File entity.
- tests: cover inbound/outbound, large-file transfer without truncation,
  and stale-dir clearing (86 passing).

web/src/app/home/pipelines/components/debug-dialog/DebugDialog.tsx

@@ -15,6 +15,7 @@ import {
   At,
   Quote,
   Voice,
+  File as FileComponent,
   Source,
 } from '@/app/infra/entities/message';
 import { toast } from 'sonner';
@@ -460,13 +461,29 @@ export default function DebugDialog({
       }
 
       case 'File': {
-        const file = component as MessageChainComponent & { name?: string };
+        const file = component as FileComponent;
+        const downloadHref = file.base64
+          ? file.base64.startsWith('data:')
+            ? file.base64
+            : `data:application/octet-stream;base64,${file.base64}`
+          : file.url || '';
+        const fileName = file.name || 'Unknown';
         return (
           <div key={index} className="my-2 flex items-center gap-2 text-sm">
             <Paperclip className="size-4" />
-            <span>
-              [{t('pipelines.debugDialog.file')}] {file.name || 'Unknown'}
-            </span>
+            {downloadHref ? (
+              <a
+                href={downloadHref}
+                download={fileName}
+                className="text-primary underline hover:opacity-80"
+              >
+                [{t('pipelines.debugDialog.file')}] {fileName}
+              </a>
+            ) : (
+              <span>
+                [{t('pipelines.debugDialog.file')}] {fileName}
+              </span>
+            )}
           </div>
         );
       }

web/src/app/infra/entities/message/index.ts

@@ -64,6 +64,8 @@ export interface File extends MessageComponent {
   name?: string;
   size?: number;
   url?: string;
+  path?: string;
+  base64?: string;
 }
 
 // Unknown component