Upgrade a workspace in place, preserving its disk

Opt-in cold start: preserves files, installed package files and home; drops memory and processes. now boots immediately; next_resume schedules a pinned target. Layered disks refresh the template base and guest; legacy disks upgrade memory only. Returns an upgrade operation. Errors: 409 already_current, legacy_disk_layout, upgrade_base_unavailable, not_upgradable, operation_in_progress, workspace_deleted; 422 not_supported_for_mode; now is admitted like a start (402/403/429 as for open). Supports Idempotency-Key. Plan concurrency: a start the plan’s concurrent_workspaces limit refuses is queued on the request (up to 10 s) and admitted as soon as one of the organization’s workspaces frees; if none does, 429 quota_exceeded with Retry-After: 2 (retryable; details limit concurrent_workspaces, limit_value, current, retry_after_seconds, plan). Nothing was created: retry after Retry-After, with the same Idempotency-Key or a new one. With Prefer: wait the queued time counts against the wait.