refactor(driver): trim compute capability response

[elezar]

Summary

Remove unused GPU availability fields from the internal compute driver capability response. GPU admission remains driver-local and is still validated when a sandbox create request asks for GPU resources.

Related Issue

None.

Changes

• Reserved the removed compute-driver capability proto fields and names.
• Stopped populating GPU capability fields from Docker, Kubernetes, Podman, VM, and test drivers.
• Documented that the capability RPC reports driver identity, version, and default image only.
• Addressed clippy fallout from the now-sync Kubernetes capability path.

Testing

• cargo check -p openshell-core -p openshell-driver-docker -p openshell-driver-kubernetes -p openshell-driver-podman -p openshell-driver-vm -p openshell-server
• cargo test -p openshell-server -p openshell-driver-docker -p openshell-driver-kubernetes -p openshell-driver-podman -p openshell-driver-vm --no-run
• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)

[elezar]

@drew would a breaking API change (simply removing the fields) be better in this case. I think we can add additional information as part of the resource requirement changes.