* refactor(docker): rename server image to gateway
Rename Dockerfile.server to Dockerfile.gateway and update all image
references from openshell/server to openshell/gateway across Helm
charts, Kubernetes manifests, mise tasks, build/deploy scripts, CI
workflows, and documentation.
The underlying Rust binary (navigator-server) is unchanged -- this
rename only affects the Docker image name and Dockerfile.
* fix: catch remaining server->gateway references in docs and comments
Drew Newberry