fix: keep auto remote-bin resolution out of command position for fish remotes (#31)

#29's auto-resolve expands to `exec "$(command -v herdr ...)"` in the
remote login shell. When that shell is fish, every connection fails:

    fish: command substitutions not allowed in command position.
    Try var=(your-cmd) $var ...

fish has no command substitution in command position at all, so any
fish-shelled ssh host breaks on status probes, terminal sessions, and
foreground polls alike.

Put `env` in front: `exec env "$(...)"` moves the substitution into
argument position, which fish (3.4+ for $(...), quoted form included)
accepts and executes identically to a POSIX sh. Verified against fish
4.8.1 and sh on a live remote; an explicit remote_bin is untouched.