feat(06-02): add sftp entry to registry, index, and rclone-conf

- Add sftp entry to BACKEND_REGISTRY with host, user, pass, key_pem fields
- Remove placeholder NOTE comment (sftp entry now present)
- Add sftp, onedrive, gcs, b2 to BACKEND_SCHEMAS in index.ts
- Add sftp, onedrive, gcs (google cloud storage), b2 to RCLONE_TYPE_MAP
- All 55 registry and rclone-conf tests pass; zero TypeScript errors

src/generators/rclone-conf.ts

@@ -11,6 +11,10 @@ const RCLONE_TYPE_MAP: Record<string, string> = {
   azureblob: 'azureblob',
   s3: 's3',
   's3-compatible': 's3',
+  onedrive: 'onedrive',
+  sftp: 'sftp',
+  gcs: 'google cloud storage',
+  b2: 'b2',
 };
 
 export function buildRcloneConf(state: WizardState): string {