feat(server): add sandbox templates

[grs]

Summary

Introduces reusable sandbox workload templates — workspace-scoped named resources that define a portable workload shape (image, environment, resources, driver config, service-level settings). Workspace admins create templates once; sandbox creation by template name inherits the workload while callers retain control over policy, providers, and labels.

This is a non-breaking alternative to #2781 that avoids tyign in the cleanup suggested in that PR. This does mean that another name was needed for the message representing sandbox templates, as SandboxTemplate is a inline section of the SandboxSpec. It also means the choice between template name or workload configuration is not explicit in the proto. These are aspects that can be cleanup in a subsequent breaking step when the time is right.

Proto: Four new RPCs (Create/Get/List/Delete SandboxTemplate), SandboxWorkloadTemplate resource with SandboxWorkloadConfig and SandboxResources, workload_template_name on CreateSandboxRequest (field 6),
SandboxWorkloadTemplateProvenance on Sandbox (field 20). No existing field numbers changed or reserved.

Server: Template CRUD handlers with workspace-scoped persistence, workspace deletion blocks on templates, governance/workload split enforcement on create-from-template (rejects inline environment, template,
resource_requirements, log_level), template-to-SandboxSpec resolution, provenance tracking, telemetry for WorkloadTemplate source.

CLI: openshell sandbox template create|get|list|delete subcommands, --template flag on sandbox create with conflicts_with_all against inline workload flags, table/JSON/names-only output formats.

SDKs: Full template CRUD across Rust, Go, Python, and TypeScript SDKs with workspace-scoped and default-workspace surfaces, mock/fake test infrastructure, and docs.

Related Issue

Fixes the sandbox template aspect of (#2680), though not the cleanup which would be treated as part of a wider effort across the proto surface.

Changes

• Add SandboxWorkloadTemplate proto resource with CRUD RPCs and authorization (admin for write, user for read)
• Server-side template resolution converts SandboxWorkloadConfig into the existing SandboxSpec/SandboxTemplate shape so compute drivers require no changes
• CreateSandboxRequest.workload_template_name triggers server-side governance validation and template lookup
• Created sandboxes carry SandboxWorkloadTemplateProvenance for audit
• Workspace deletion now blocks on existing templates
• CLI template subcommands with service-level flags (--ready-within, --max-burst)
• Rust SDK: curated methods on OpenShellClient and WorkspaceScopedClient, raw proto type aliases
• Go SDK: SandboxTemplateInterface with domain types, converter layer, fake client
• Python SDK: SandboxTemplateClient with from_sandbox_client constructor, Sandbox context manager gains template_name
• TypeScript SDK: SandboxTemplateClient wired into OpenShellClient.sandboxTemplates
• E2E test covering template create, get, list, and sandbox-create-from-template
• Published docs updated with template CLI and Python SDK examples

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

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

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