fix(scripts): answer a missing --files with a usage message, not a stack trace

[os-elon]

Fixes#113

Run without --files, this gate threw an uncaught Error: Node printed the source frame plus a seven-line stack and its own version banner. The message was right and the exit code was already 1 — the shape was wrong. A stack trace says "this script is broken" when the truth is "you called it wrong", and it misled exactly that way once: a dispatch prompt listed the bare command as a check to run clean, and the trace read as a regression until it was diffed against main.

Argument validation now throws a UsageError, and the entry point catches that class only — message and usage line on stderr, no stack, still exit 1. Every other throw is rethrown untouched, so an unparseable i18n.ts, an unreadable list file or a bug in the check keeps the stack a real fault needs.

Missing required argument — before and after

Before:

$ node .github/scripts/check-translation-ownership.mjs
file:///home/user/objectos-issue-113/.github/scripts/check-translation-ownership.mjs:65
if (!listFile) throw new Error('--files path is required');
^
Error: --files path is required
at main (file:///…/check-translation-ownership.mjs:65:24)
at file:///…/check-translation-ownership.mjs:116:1
at ModuleJob.run (node:internal/modules/esm/module_job:343:25)
at async onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:665:26)
at async asyncRunEntryPointWithESMLoader (node:internal/modules/run_main:117:5)
Node.js v22.22.2
[exit 1]

After:

$ node .github/scripts/check-translation-ownership.mjs
✗ --files path is required
usage: node .github/scripts/check-translation-ownership.mjs --actor login --files path
--actor login PR author login (workflows pass github.event.pull_request.user.login)
--files path file listing one changed path per line (git diff --name-only)
[exit 1]

(The two placeholders are angle-bracketed in the real source and the real output; GitHub's body sanitizer eats that token, which is also why the card's own quote of line 65 is missing it.)

Same result for the other three shapes of the same mistake: no arguments at all, --actor alone, --files with no value, and --files= with an empty value.

It still fails. Exiting 0 on a missing required argument would turn a misinvocation into a silent pass, which is the species of dishonest green this repo removed from the test step in #112.

Correct invocations are byte-identical

Twelve invocations were recorded on main and again on this branch and diffed: four inert-path shapes (TRANSLATION_BOT_LOGIN unset), four enforcing-path shapes with the variable set (human clean, human with artifacts rejected, bot artifacts-only accepted, bot mixed rejected), the --files= spelling, and three shapes with --actor absent. Zero diff lines across all of them — the only changes in the transcript are the four usage-error shapes above.

A genuine internal failure keeps its stack, verified with a --files path that does not exist: same node:fs:440 source frame, same ENOENT properties, same version banner, only the line numbers moved as the file grew.

Deliberately not in this PR

• --actor is left alone. It is documented as required but implemented as optional ((value('actor') ?? '').trim()), so omitting it does not crash — it silently classifies the run as a human PR. That is a different defect from this card's, and requiring it would flip an invocation that exits 0 today into exit 1 through the inert path that The ownership check is inert on main until TRANSLATION_BOT_LOGIN is set #68 owns. Measured and filed as check-translation-ownership.mjs accepts a missing --actor silently and then judges the PR anyway #116 with options.
• No self-test mode, so tools/ci-scripts/run-self-tests.mjs and its SELF_TESTED list are untouched — the runner's own comment names this script as deliberately not run there. Its guard was exercised anyway: turbo run test executed for real on this edit (.github/scripts/** is an input to that task) and passed.
• No change to ownership logic, the TRANSLATION_BOT_LOGIN inertness, or the workflow. .github/workflows/translations.yml passes both flags, so the one real caller never reaches the changed path.

Gates, at b48bc8c

gate	result
pnpm install --frozen-lockfile	0
pnpm turbo run type-check --continue	0
pnpm turbo run build	0
pnpm turbo run test	0 — first run after the edit: 0 cached, 1 total, real execution, 20 self-test cases
node .github/scripts/check-translations.mjs	0, gate passed
node .github/scripts/check-translation-ownership.mjs --actor os-elon --files changed.txt	0, inert report
node .github/scripts/check-translation-output.mjs --files changed.txt	0, 127 pre-existing findings reported