Re-couple universal image to shared UBUNTU_VERSION once devcontainers/universal publishes a resolute tag

### Problem Description

The `universal` image is built from `mcr.microsoft.com/devcontainers/universal:${UBUNTU_VERSION}`, which normally tracks the shared `UBUNTU_VERSION` single source of truth in `scripts/images.sh`. When migrating the images to Ubuntu 26.04 (`resolute`), Microsoft's `devcontainers/universal` image does not yet publish a `resolute` tag. Its latest codename tag is `noble` (24.04).

As a result, `universal` was temporarily decoupled from the shared `UBUNTU_VERSION` and pinned to `:noble` so the migration to `resolute` for the other images could ship without breaking the `universal` build.

### Desired Solution

Once Microsoft publishes a `resolute` (26.04) tag for `mcr.microsoft.com/devcontainers/universal`, re-couple the `universal` Dockerfile to the shared `UBUNTU_VERSION` build arg:

```dockerfile
ARG UBUNTU_VERSION=resolute
FROM mcr.microsoft.com/devcontainers/universal:${UBUNTU_VERSION}
```

and remove the temporary pin comment.

### Verification

```
docker manifest inspect mcr.microsoft.com/devcontainers/universal:resolute
```

When this succeeds, the tag is available upstream.

> 🤖 This response was generated by Coder Agents.