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

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 #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 #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).