fix the silent death after installing zsh

Reported from a fresh Hetzner VPS: the run stopped dead right after apt finished
installing zsh, printing nothing at all — just install.sh's "machine setup did not
finish".

install_oh_my_zsh carried a comment saying it "Returns 0 whatever happens". It did
not. Under `set -e` a failing command inside a function aborts the SHELL at that
line when the function is called plainly; `return 0` underneath is never reached.
The command is also `>/dev/null 2>&1`, so the cause was invisible — which is why
the transcript just ends.

`|| true` is what actually makes it non-fatal. The file already uses that idiom
correctly in four other places, so this was a slip rather than a misunderstanding.

set_login_shell had the identical bug on `chsh`, which the same run would have hit
on the very next question. Fixed differently and deliberately: `|| true` there
would let the caller announce a login shell that was never set, so it returns
chsh's real status and the CALLER guards the call — which is also what keeps set -e
out of it. A refusal now reports, names the manual chsh command, and carries on,
because a machine with zsh installed and bash at login still works.

Does not explain WHY oh-my-zsh failed on that host — the output was discarded. It
will now say "oh-my-zsh did not install" and continue, which is enough to see it.

Verified: bash -n on both files, and a reduced case proving broken() exits 1 while
fixed() survives.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/setup/machine-setup/lib/dev.sh

@@ -92,27 +92,36 @@ oh_my_zsh_installed() { [[ -d "${USER_HOME}/.oh-my-zsh" ]]; }
 install_oh_my_zsh() {
   # The installer refuses to run unattended over an existing install, so this is
   # only ever called when there is none.
+  #
+  # ── `|| true` is what makes this non-fatal, NOT the `return 0` below ──
+  #
+  # It used to be `return 0` alone, with a comment claiming the function returned
+  # zero whatever happened. It did not. Under `set -e` a failing command inside a
+  # function aborts the SHELL at that line when the function is called plainly —
+  # `return 0` is never reached. So a machine where this curl or the installer
+  # failed died here, silently, because the output is redirected: the run just
+  # stopped after apt finished installing zsh, with nothing said. Observed on a
+  # fresh Hetzner VPS, 2026-08-14.
   sudo -H -u "$USERNAME" sh -c \
-    "$(curl -fsSL https://raw.githubusercontent.com/ohmyzsh/ohmyzsh/master/tools/install.sh)" "" --unattended >/dev/null 2>&1
-  # Returns 0 whatever happens. This is an optional improvement, and a
-  # function that ends on a failing command is fatal under `set -e` when it
-  # is called as a plain command — which would abort the remaining sections
-  # over something the run could simply report. The caller checks the outcome.
+    "$(curl -fsSL https://raw.githubusercontent.com/ohmyzsh/ohmyzsh/master/tools/install.sh)" "" --unattended >/dev/null 2>&1 ||
+    true
+  # Belt and braces: `|| true` above already makes the last command succeed, and
+  # this states the contract for anyone adding a line beneath it.
   return 0
 }
 
 # `chsh` is what actually changes the login shell. Asked separately from
 # installing zsh, because having a shell available and being handed it at every
 # login are different decisions.
+# Reports whether chsh worked, rather than swallowing it. The same `set -e` trap as
+# install_oh_my_zsh applies — a bare `chsh` that fails kills the run at this line —
+# but here the answer matters: the caller announces the new login shell, and `|| true`
+# would have it announce one that was never set. So the status comes back and the
+# CALLER guards the call, which is also what keeps set -e out of it.
 set_login_shell() {
   local shell="$1"
   grep -qxF "$shell" /etc/shells || echo "$shell" >>/etc/shells
-  chsh -s "$shell" "$USERNAME"
-  # Returns 0 whatever happens. This is an optional improvement, and a
-  # function that ends on a failing command is fatal under `set -e` when it
-  # is called as a plain command — which would abort the remaining sections
-  # over something the run could simply report. The caller checks the outcome.
-  return 0
+  chsh -s "$shell" "$USERNAME" >/dev/null 2>&1
 }
 
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