refactor(app): extract composer-actions module with pure tests (#720)

Move composer cancel/queue/attachment/dispatch orchestration out of
composer.tsx into a React-free actions module. The module takes its
dependencies (send client, queue writer, stream writer, attachment
persister, image picker) as injected ports, so the new
composer-actions.test.ts can drive every action with inline fakes —
zero vi.mock of internal modules, zero JSDOM, zero React.

The old composer.test.tsx (heavy mocked component test) is removed
and replaced with composer-actions.test.ts (31 unit tests).

Also extract isWorkspaceAttachment / userAttachmentsOnly /
workspaceAttachmentToSubmitAttachment from composer-workspace-attachments.tsx
into a sibling .ts so the actions module (and composer-attachments.ts)
can use them without dragging React/RN/lucide into a pure test graph.

packages/app/src/components/composer-attachments.ts

@@ -1,5 +1,8 @@
 import type { AttachmentMetadata, ComposerAttachment } from "@/attachments/types";
-import { composerWorkspaceAttachment } from "@/attachments/composer-workspace-attachments";
+import {
+  isWorkspaceAttachment,
+  workspaceAttachmentToSubmitAttachment,
+} from "@/attachments/workspace-attachment-utils";
 import type { AgentAttachment } from "@server/shared/messages";
 import { buildGitHubAttachmentFromSearchItem } from "@/utils/review-attachments";
 
@@ -18,8 +21,8 @@ export function splitComposerAttachmentsForSubmit(attachments: ComposerAttachmen
       continue;
     }
 
-    if (composerWorkspaceAttachment.is(attachment)) {
-      const workspaceAttachment = composerWorkspaceAttachment.toSubmitAttachment(attachment);
+    if (isWorkspaceAttachment(attachment)) {
+      const workspaceAttachment = workspaceAttachmentToSubmitAttachment(attachment);
       if (workspaceAttachment) {
         reviewAttachments.push(workspaceAttachment);
       }