feat: paste clipboard images into mirror panes via ctrl+v (#39)

herdr's native image paste (remote_image_paste) only exists in
herdr --remote; a mirror pane's terminal-session stream carries no
image message, so a clipboard image had no way across. Intercept a
lone ctrl+v in the pane wrapper, read the local clipboard (pngpaste/
AppleScript on macOS, wl-paste/xclip on Linux), upload over the pane's
own transport (ssh reusing the daemon's ControlMaster, or docker exec)
to ~/.cache/herdr-mirror/pastes/ on the remote, and paste the absolute
path as bracketed text for the remote agent to read. No image on the
clipboard forwards the ctrl+v unchanged.