feat(desktop+gateway): remote-gateway file attachments via file.attach

@file: attachments now work when the desktop is connected to a remote
gateway. Previously a referenced file resolved to a client-disk path the
gateway couldn't see, so context_references rejected it with "path is
outside the allowed workspace" and the agent never saw the file.

Adds a file.attach RPC (sibling to the existing image.attach_bytes /
pdf.attach byte-upload pipeline): the desktop uploads the file bytes, the
gateway stages them into <workspace>/.hermes/desktop-attachments/ and
returns a workspace-relative @file: ref that resolves cleanly. Local mode
passes the path directly; a gateway-visible file outside the workspace is
copied in; an in-workspace file is referenced as-is with no copy.

Consolidates the file-sync design from #38615 (LeonSGP43) and the
host-file-staging idea from #33455 (Carry00), rebased onto the
image/PDF remote-media helpers already on main.

Co-authored-by: LeonSGP43 <cine.dreamer.one@gmail.com>

apps/desktop/src/store/updates.ts

@@ -88,7 +88,8 @@ function isUpdateToastSnoozed(): boolean {
 // Must match tui_gateway's DESKTOP_BACKEND_CONTRACT that this build was written
 // against. The backend reports its own value in session runtime info; a lower
 // value (or none — a pre-GUI checkout) means GUI<->backend skew.
-const REQUIRED_BACKEND_CONTRACT = 1
+// v2: requires the file.attach RPC (remote-gateway non-image file upload).
+const REQUIRED_BACKEND_CONTRACT = 2
 const SKEW_TOAST_ID = 'backend-contract-skew'
 
 /**