muxg: agent forwarding (-A) for a pane

open   by a73x

The TUI offers an ssh agent to a session with `-A`: the wall client answers the daemon's per-session agent socket, latest-active `-A` client wins, frames capped at `agent_data_max`. Nothing under `src/gui/` mentions an agent, so a session opened from `muxg` on a remote daemon has no `SSH_AUTH_SOCK` that works, and git or ssh inside it fails where the same session under `mux -A` succeeds.

Wanted: a per-pane `-A` equivalent in the GUI (a workspace flag persisted with the pane, or a global toggle), reusing the client row's agent relay so the frame contract and the cap are not reimplemented. A gate on retiring the wall.