Retire the WSL2 path for persistent PTY live legs on Windows #537

From the v2.1.x pin on (#532), upstream's opencode-pty daemon (@opencode-ai/pty 0.3.1) ships a win32 binary, and the server enables persistent PTYs on Windows. #532's CI proved it: the persistent PTY live round trip and SubmitAsync pass on Windows on net10.0, net11.0 and net472 through the daemon-present arm, and the test gate now expects the daemon on every platform.

The WSL2 path existed only because there was no win32 daemon, so it is now a workaround without a reason:

• docs/engineering/developing-on-windows.md, "Live legs that need a Linux-hosted server": its premise, "no win32 package exists at the pinned upstream commit", is false at the pin. The WSL2 clone recipe, the node_modules clobbering warning tied to it, and the "exact-pin discipline" paragraph exist for that leg.
• tests/OpenCode.Sdk.Sandbox/README.md, "Live legs against a server on another host": it names the WSL2 persistent PTY leg as "the standing case" because "the opencode-pty daemon ships no win32 package".

What stays

• The fixture's external-endpoint mode (OPENCODE_SDK_TESTS_ENDPOINT / OPENCODE_SDK_TESTS_PASSWORD). It is general: it runs the suite against any server hosted elsewhere, so only its WSL2-specific wording changes.
• OPENCODE_SDK_TESTS_PTY_DAEMON=0|1. It still selects either arm, for example for a host whose server cannot start the daemon.

Done when

• Neither document presents WSL2 as needed for persistent PTY live legs on Windows. The developing-on-windows section either goes, or keeps only what still holds on its own (for example, "Driving WSL from a Windows shell", if other legs use it).
• No other document or comment says the daemon ships no win32 package. Checked with a repository search for win32 and WSL2 near "persistent" and "daemon".
• The fixture's remarks keep describing external-endpoint mode without implying that WSL2 is required.