fix(update): report failure when systemctl will not start the update timer

[clawedassistant26]

What

moshcode update --timer --install writes the systemd units, runs systemctl daemon-reload and systemctl enable --now moshcode-update.timer, then always prints checking on a schedule now and exits 0 — even when the enable fails.

On a host without systemd (a container, WSL, macOS) or without root, the timer never starts. The user is told their machine is auto-updating on a schedule when it is not. Silent, and exactly the kind of promise you only discover was empty when the update you were counting on never happened.

Two layers were dropping the failure:

• bin/moshcode.mjs wired runner as execFile(cmd, args, () => res({ ok: true })) — the callback ignores execFile's error and always resolves { ok: true }, so selfUpdateCommand could never see a failure.
• src/selfupdate.mjs ignored the runner results entirely and printed success unconditionally.

Fix

• Surface execFile's error: (err) => res({ ok: !err }).
• In selfUpdateCommand, when daemon-reload or enable reports ok:false, tell the user the timer is not scheduled yet and return 1 instead of claiming success.

The existing tests already inject runner: () => ({ ok: true }), so the { ok } contract is the intended one — this change just honors the failing side of it.

Evidence (real selfUpdateCommand, systemctl enable failing)

Before:

exit code: 0
units written: 2
checking on a schedule now. `systemctl list-timers moshcode-update` to see when.

After:

exit code: 1
units written: 2   (still written — we just do not lie about the timer)
moshcode update: wrote the units but systemctl could not start the timer — it is not checking on a schedule yet. run `systemctl enable --now moshcode-update.timer` as root.

Tests

New regression test in test/selfupdate.test.mjs: with systemctl enable returning ok:false, asserts exit 1, units still written, no "checking on a schedule now", and the failure message. Fails before this change, passes after.

Controls unchanged: the ok:true install path still prints checking on a schedule and returns 0.

Full suite green: 1215 tests, 1002 pass, 0 fail, 213 skipped. Two source files + one test file; generated package-lock.json removed before commit.