docs(sdk): action handlers must i18n what they throw

[MattDHill]

The i18n() convention in actions.md carved out thrown errors:

Thrown errors are the exception. throw new Error(...) messages are developer-facing diagnostics that surface in logs and stack traces, not translated UI copy — leave them as plain strings and do not wrap them in i18n().

For an action handler the premise is false. ActionService.execute catches the error and renders the message straight into an alert dialog labelled "Error" — action.service.ts:78-81. That alert is the only thing telling the user the action failed, so the guide was directing packagers to leak English into it in every non-default locale. Found while auditing ppq-private-mode-startos, whose "An API key is required…" message is exactly this case; the CLI shows the same string (Action Failed: Error: An API key is required…).

Wrapping it works end to end, which the old wording also left unclear:

• setupI18n resolves eagerly against the container's LANG and returns a finished string (i18n/index.ts:29,62-64), so what crosses the wire is already translated.
• i18nPipe looks the string up in ENGLISH and falls through to the input verbatim on a miss (i18n.pipe.ts:16-18), so a package-translated message renders as-is rather than as a missing-key artifact.

The exception is real for throws that aren't in an action. setupMain / setupInit / migration failures land in statusInfo.error and render as a Service Launch Error card next to Rebuild and Uninstall buttons (error.component.ts) — a crash report, not copy. So the rule is split by throw site rather than deleted.

init.md:233's plain-string throw new Error('Bootstrap failed') sits in runUntilSuccess bootstrap and stays correct under the new wording. No other page states the old rule.

Targeting live-docs rather than master: the wrong sentence is on docs.start9.com now, and what it gets wrong is how shipped StartOS already behaves — not an unreleased SDK surface — so this is a correction to the published site rather than a doc riding along with its code. docs-backport.yml pushes the same commit to master on merge, so it lands in both without being written twice. No CHANGELOG.md entry: book-only, no change to the SDK's shipped surface.