Go ui_api: add `PUT /v1/environments/:env/ds/nodes/:id` and `DELETE /v1/environments/:env/ds/nodes/:id`

As of 2026-05-14, the Go ui_api now supports `POST /v1/environments/:env/ds/nodes` for design-studio node creation (huge unlock — thank you). But for **per-node updates and deletes**, agents still have to fall back to:

- `PATCH /design-studio/api/workspaces/:ws/nodes/:id` on the legacy Design Studio surface for updates, or
- `POST /v1/environments/:env/ds/node_actions/bulk_delete` with a single-element `node_ids` array for deletes.

Both work (the legacy surface now accepts service-account JWTs, which it didn't before), but the surface split is awkward — the create returns from the Go ui_api, then you switch hosts/paths to mutate the same resource.

### Suggested

Add `PUT /v1/environments/:env/ds/nodes/:id` (or `PATCH`) and `DELETE /v1/environments/:env/ds/nodes/:id` to the Go ui_api so a node's full CRUD lives on one surface.

### Why it matters

Today's split makes the `cio` agent skill more confusing (the `ds` resource only shows create on its schema; agents have to learn the legacy path separately). Consolidating would let `cio schema ds` describe the full lifecycle.