The first commit made `--shell` reach the spawn, but only a LOCAL win32
execution host applies it: `spawn-options` gates the override on
`process.platform === 'win32' && !args.connectionId`. So `--shell cmd.exe`
against an SSH-routed worktree, or against a macOS/Linux host, still returned a
healthy terminal running that host's default shell — the same
indistinguishable-from-success reply the capability gate exists to prevent, one
layer down.
Refuse instead, before anything spawns. The check sits at the top of
`resolveAgentTerminalCreateOptions`, which every create lane funnels through, so
neither lane has to remember it; the desktop lane additionally refuses a
worktree-less create, which has no execution host to resolve a shell on.
An SSH host's platform and installed shells are not visible to this runtime, and
a POSIX host has no Windows shell to pick. Neither can honour the request, and
saying so is the whole point of the flag.
Docs and the CLI spec now say "refused", not "ignored".