feat(docs): Add documentation for OpenShell Kubernetes RBAC requirements

[TaylorMutch]

Problem Statement

There is no documentation for the Kubernetes RBAC permissions required by the OpenShell Gateway. Operators deploying to existing clusters — especially multi-tenant environments — cannot easily determine what access the Gateway service account needs without reading the Helm chart templates directly.

Proposed Design

Add a dedicated RBAC reference page to the docs (e.g., docs/deployment/kubernetes-rbac.mdx) that covers:

1. What the Helm chart creates by default — ServiceAccount, Role, and RoleBinding installed by deploy/helm/openshell/templates/.

2. Full permission table — document every rule in the Role:

   API Group	Resource	Verbs	Scope	Purpose
   agents.x-k8s.io	sandboxes, sandboxes/status	create, delete, get, list, patch, update, watch	Namespaced	Sandbox lifecycle management
   "" (core)	events	get, list, watch	Namespaced	Sandbox event observation
   "" (core)	nodes	get, list	Cluster	GPU capacity validation (required only when GPU sandboxes are enabled)

3. GPU deployments — note that GPU sandbox support requires an additional ClusterRole binding so the Gateway can list nodes cluster-wide to check allocatable GPU resources. Non-GPU deployments do not need this permission.

4. Bring-your-own ServiceAccount — how to disable the auto-created ServiceAccount (serviceAccount.create: false) and supply an existing one (serviceAccount.name).

5. Multi-tenant guidance — how to scope the Gateway to a specific namespace and disable cluster-level resource access when GPU support is not needed.

Alternatives Considered

Embedding RBAC details in the main deployment guide rather than a dedicated page would be more concise, but a standalone reference page is easier to link to from error messages or support docs and allows the table to expand without cluttering the deployment walkthrough.

Agent Investigation

• The Helm Role at deploy/helm/openshell/templates/role.yaml covers agents.x-k8s.io/sandboxes and core events — both namespaced.
• The driver calls Api::<Node>::all() at crates/openshell-driver-kubernetes/src/driver.rs:184–192 exclusively for GPU capacity checks. This is only exercised when a user requests a GPU sandbox (validate_sandbox_create, driver.rs:195–207). It requires nodes: [get, list] at cluster scope, which the current Helm chart does not include. The Helm chart likely needs a conditional ClusterRole/ClusterRoleBinding gated on a gpu.enabled or similar value.
• The TLS secret (client_tls_secret_name) is referenced only in pod volume specs — the Gateway does not read it directly via the Secrets API, so no secrets: get permission is needed.

Checklist

• I've reviewed existing issues and the architecture docs
• This is a design proposal, not a "please build this" request