fix(core): Location liveness check stats the server fs for workspace Locations

What

`LocationServiceMap` keeps a Location alive based on a process-local `existsSync(ref.directory)` (see `packages/core/src/location-services.ts` / `location-service-map.ts` existence check). For a workspace-backed Location whose directory only exists inside the sandbox (e.g. `/workspace` in a Modal image, DO-hosted sessions), this check consults the wrong filesystem: the Location can be evicted or never admitted even though the placement is healthy.

Fix direction

Skip the local existence check when `location.workspaceID` is set. Workspace liveness is owned by the workspace row plus the provider binding; a cheap substitute is "workspace row exists", which is already a point read. Do not probe the sandbox for liveness — that would provision lazily-idle workspaces.

Acceptance

• A workspace Location whose directory does not exist on the server host builds and survives.
• Local Location eviction behavior unchanged.

Found while auditing boot-cycle process-vs-workspace resolution after #44526. Mechanical fix; no design dependency.

Start with the existence check in packages/core/src/location-services.ts and location-service-map.ts, then trace how workspaceID is handled for a Location. Confirm that workspace-backed Locations do not depend on the server filesystem while local Location eviction remains unchanged. Use the issue’s acceptance criteria to verify that a workspace directory absent on the host can still build and remain alive.