feat(gpu): disable NFD/GFD and remove nodeAffinity from device plugin chart

[elezar]

Summary

• Disables GPU Feature Discovery (GFD) and Node Feature Discovery (NFD) DaemonSets in the NVIDIA device plugin HelmChart
• Overrides the device plugin's default nodeAffinity to {} so the DaemonSet schedules unconditionally on the single-node gateway without requiring NFD/GFD labels (feature.node.kubernetes.io/pci-10de.present=true or nvidia.com/gpu.present=true)
• Updates architecture docs and debug skill to reflect the change

Related Issue

N/A — pure declarative simplification with no new runtime code paths.

Changes

• deploy/kube/gpu-manifests/nvidia-device-plugin-helmchart.yaml: disable gfd/nfd, add affinity: {} override, update comment block
• architecture/gateway-single-node.md: update GPU Enablement section to explain NFD/GFD are disabled and why
• .agents/skills/debug-openshell-cluster/SKILL.md: add troubleshooting entry for lingering NFD/GFD DaemonSets on clusters deployed before this change

Testing

• openshell gateway start --gpu — device plugin DaemonSet reaches Ready (1/1)
• kubectl get daemonset -A | grep -E 'nfd|gfd|node-feature' — no output
• kubectl get node -o jsonpath='{.items[0].status.allocatable}' — nvidia.com/gpu key present
• mise run test — no regressions

Checklist

• Conventional commit message
• mise run pre-commit passed
• Architecture docs updated
• Debug skill updated per cluster infra change instructions in AGENTS.md