feat(desktop-main): SDK bootstrap of S3 tfvars bucket

[CoderCoco]

Child of #139 (epic: first-run wizard + credentials UX). Authoritative scope in the design spec.

Scope

Programmatic creation of the versioned tfvars bucket per #84/#85. Lifecycle: 90-day version retention. Idempotent.

Acceptance

• Bucket exists with versioning + lifecycle policy.
• Used as the canonical RemoteFileStore.