OpenSandbox Windows guest profile (wait on Go SDK Platform field)

[zpzjzj]

Once OpenSandbox-team/OpenSandbox#921 merges (Go SDK gains PlatformSpec{OS, Arch} on CreateSandboxRequest and SandboxInfo), wire up Windows guest sandboxes:

• runtime.Config accepts a Platform field (already documented as the future shape in docs/guide/windows.md).
• OpenSandboxRuntime passes platform.os = windows / arch = amd64 through to the OpenSandbox API.
• OpenSandboxRuntime.TargetGOOS() (or its replacement once Architecture: fold TargetGOOS into a Runtime.Shell() abstraction #44 lands) returns the requested guest OS instead of the current hardcoded "linux".
• Add an opt-in e2e job (E2E (opensandbox runtime, Windows guest)) gated on the OpenSandbox endpoint actually serving Windows guests.

Blocked on:

• Upstream Go SDK PR: OpenSandbox-team/OpenSandbox#921 (typed PlatformOS / PlatformArch enums + PlatformSpec). Filed during PR feat: first-class Windows support across CLI, runtime, and judges #33; pending upstream review.

Coordinates with #44: once Runtime.Shell() lands, this issue's "return non-linux GOOS" wiring should happen there, not on the standalone TargetGOOS() method.

Tracked from: PR #33 self-review (deferred from the original scope of #31, which proposed both none + opensandbox Windows support).