review: use RunWithContext for backend prompt; clarify validator doc

Address PR #1661 bot review:

• promptCheckpointBackend now runs via RunWithContext(ctx) and routes failures through handleFormCancellation, matching the setup-family convention (setup_import.go). Ctrl+C / cancelled context now abort cleanly and keep the default backend instead of surfacing a generic wrapped error.
• Clarify ValidatePrimaryBackend doc: unknown types error with the registered type list; non-git-backed types are rejected separately.