Astro i18n setup (scaffold-time, opt-in)

Wires turnkey internationalization into the new-website Astro overlay. Opt-in: the default scaffold stays single-locale. Run it either at scaffold time for a 2+-language launch (new-website §1 Q4), or later when you add a second language to an existing single-locale site — see Phased rollout below.

Safe vs unsafe: because the default locale stays unprefixed (/, not /en/), adding a secondary prefixed locale (/de/…) is purely additive — it never moves an existing URL. What DOES break links/SEO is moving the default locale itself off / (e.g. retrofitting / → /en/); this skill never does that, so keep prefixDefaultLocale: false. A full re-prefixing of the default is a migration, not this skill.

Routing model (house default): clean default locale, prefixed others — / and /about are the default language (marked x-default); other languages get a prefix (/de/, /de/about). prefixDefaultLocale: false. This keeps the default-locale URLs identical to a single-locale build, so nothing regresses.