@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>
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