fix(tuxcloud): run install step via script, not inline POSIX shell

[nosduco]

Fixes the Command [...] failed on the tuxcloud step in #1.

Cause

Dotbot runs shell commands through whatever $SHELL says:

# dotbot/src/dotbot/util/common.py
executable = os.environ.get("SHELL")
return subprocess.call(command, shell=True, executable=executable, ...)

.tmux.conf:2 sets default-shell /bin/fish, so a tmux pane exports SHELL=/bin/fish. The inline if ... then ... fi was handed to fish, which does not accept POSIX if-syntax:

fish: Missing end to balance this if statement

Reproduced with the exact string parsed from install.conf.yaml, through dotbot's own shell_command:

SHELL=/bin/bash   exit = 0
SHELL=/bin/fish   exit = 127

This passed pre-merge verification because those checks ran from a bash context. My mistake: I validated the step by hand instead of through the shell ./install actually uses.

Fix

Logic moves to scripts/install-tuxcloud-mount.sh with a #!/bin/bash shebang, which pins the interpreter no matter what the caller's shell is. install.conf.yaml just invokes it, with a comment recording why.

The Nautilus bookmark step is folded into the same script. It happened to be fish-compatible, but only by luck.

Also drops the || true that was swallowing the enable --now result. The server is LAN-only, so that genuinely fails off-LAN; it now prints a warning and leaves the unit enabled for next boot instead of reporting success.

Verification

Run dotbot-style under each shell, twice, for idempotency:

SHELL=/bin/bash   run1 exit = 0    run2 exit = 0
SHELL=/bin/fish   run1 exit = 0    run2 exit = 0
SHELL=/bin/sh     run1 exit = 0    run2 exit = 0

service: active    bookmark entries: 1    mount entries: 43

Related, not fixed here

The other ./install failure (voyager-cmdline-tweaks.sh) is a separate, pre-existing bug: dotbot's shell plugin ignores if:. Only the link plugin implements it (link.py:51,56); shell.py reads just command/description/stdin/stdout/stderr/quiet. So every host-gated shell command runs on every host. All eight voyager system files are currently installed on nighthawk, and voyager-power-state.service is enabled on the desktop. Worth its own PR - needs a decision on approach.