User-data migration shim: detectPriority + branches

## Что это ## Контекст user-data-migration R1-R9: detectPriority (\~/.rox-agent/ → \~/.rox/), migrateUserDataIfNeeded() в config/, copy non-destructive (legacy preserved), marker file \~/.rox/.migrated-from-rox. ## Модель данных * MigrationResult { decision: 'noLegacy'|'legacyOnly'|'bothExist'|'alreadyMigrated'; copiedBytes?: number; warnings: string\[\] } * MigrationLogger contract { info, warn, error } ## Критерии приёмки - [ ] 4 state-machine cases работают: no-legacy, legacy-only, both-exist, already-migrated - [ ] Already-migrated branch checked first (idempotent) - [ ] Conflict (both exist) → warn no-copy no-marker - [ ] fs.cpSync с verbatimSymlinks - [ ] Marker file пишется атомарно - [ ] Node 22+ vs older graceful ## Риски * Disk-space double usage during copy → проверять free space * Windows long-paths → enable longPath support ## Статус Это задача из текущего backlog'а ROX.ONE (Linear). Текущий статус в Linear: `Triage`. Метки: `История` / `Story`, `Фича` / `Feature`. ## 🔗 Linear - [PZD-128](https://linear.app/kuhjie/issue/PZD-128/istoriya-user-data-migration-shim-detectpriority-branches) — backing ticket - Parent epic: [PZD-122](https://linear.app/kuhjie/issue/PZD-122)