docs: update gateway deployment architecture

[drew]

Summary

Rewrite the OpenShell documentation and architecture references away from the embedded k3s cluster image model and toward a gateway plus compute-platform model.

Related Issue

None.

Changes

• Reframed published docs around registered gateway endpoints backed by Docker, Podman, Kubernetes, or MicroVM compute platforms.
• Kept the Helm chart as the Kubernetes deployment path while removing cluster image/bootstrap publishing language from docs.
• Reworked architecture docs for gateway deployment, packaging, security, system diagrams, sandbox image delivery, and inference/gateway wording.
• Updated agent and contributor docs, including troubleshooting skills, to diagnose gateway deployment/runtime issues instead of k3s cluster bootstrap.

Testing

• mise run docs
• helm lint deploy/helm/openshell
• git diff --check
• mise run pre-commit
• Unit tests added/updated (docs only)
• E2E tests added/updated (docs only)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated

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[cgwalters]

Only skimmed, just approving the direction! Will be a big improvement.

[github-actions]

🌿 Preview your docs: https://nvidia-preview-pr-1108.docs.buildwithfern.com/openshell