Get registry credentials for pushing agent images

Get short-lived credentials for pushing images to a namespace's registry.

Each namespace has its own isolated registry repository. The token returned
can ONLY push images to the specified namespace's repository.

The CLI uses this endpoint to authenticate with Docker before pushing
agent images. Returns a token valid for approximately 1 hour.

**Usage with Docker:**
```bash
docker login {registry_url} -u {username} -p {token}
docker push {registry_url}/{repository}/{agent}:{tag}
```