feat: stream files as binary frames over WebSocket (#659)

Replace base64-in-JSON file transfers with binary WebSocket frames to
unblock the JS thread when assistant markdown messages contain images.
Receive cost (JSON.parse over multi-MB base64) and persist cost (base64
re-encode to disk) both removed.

- Split daemon-client `exploreFileSystem` into `listDirectory` + `readFile`;
  `readFile` returns `{ bytes, mime, size, modifiedAt }`. Encoding stays
  private to the seam.
- New `packages/server/src/shared/binary-frames/` module hosts terminal
  frames (moved from `terminal-stream-protocol.ts`) and new file-transfer
  opcodes (`FileBegin` / `FileChunk` / `FileEnd`) with request-id correlation.
- Add optional `acceptBinary` to `FileExplorerRequestSchema`. New daemons
  emit binary frames when set; legacy JSON path remains for old clients.
  Compat fallback lives only inside `readFile`.
- New `persistAttachmentFromBytes` writes bytes directly: native via
  `expo-file-system` `File.write(Uint8Array)`, web via Blob -> IndexedDB,
  desktop via new `write_attachment_bytes` IPC. `persistAttachmentFromBase64`
  removed.

packages/server/src/shared/messages.test.ts

@@ -1,5 +1,5 @@
 import { describe, expect, test } from "vitest";
-import { SessionOutboundMessageSchema } from "./messages.js";
+import { FileExplorerRequestSchema, SessionOutboundMessageSchema } from "./messages.js";
 
 function workspaceDescriptor(overrides: Record<string, unknown> = {}) {
   return {
@@ -126,3 +126,38 @@ describe("workspace descriptor message compatibility", () => {
     });
   });
 });
+
+describe("file explorer request compatibility", () => {
+  test("acceptBinary is optional for old clients and accepted for new clients", () => {
+    expect(
+      FileExplorerRequestSchema.parse({
+        type: "file_explorer_request",
+        cwd: "/repo/app",
+        path: "image.png",
+        mode: "file",
+        requestId: "req-old",
+      }),
+    ).toEqual({
+      type: "file_explorer_request",
+      cwd: "/repo/app",
+      path: "image.png",
+      mode: "file",
+      requestId: "req-old",
+    });
+
+    expect(
+      FileExplorerRequestSchema.parse({
+        type: "file_explorer_request",
+        cwd: "/repo/app",
+        path: "image.png",
+        mode: "file",
+        requestId: "req-new",
+        acceptBinary: true,
+      }),
+    ).toMatchObject({
+      type: "file_explorer_request",
+      requestId: "req-new",
+      acceptBinary: true,
+    });
+  });
+});