Docs: Windows users have no setup path — document WSL2 as the supported route #189

Background

Several Windows users on Slack hit the same wall independently:

"Ser at det ikke er mulig å slå på sandboxing i github copilot plugin i pycharm på windows (trenger en insider build). cplt fungerer ikke på windows. copilot CLI sin sandbox trenger også en insider build. Så da kan man i praksis kun bruke vscode på windows?"

"Jeg savner bedre guideline for hvordan sette opp på en windows-maskin. Jeg tror jeg har fått satt opp Ubuntu gjennom wsl.exe --install, etterfulgt av installasjon av gh, gh auth login, npm install -g @github/copilot, osv."

Sandboxing is now a requirement for agentic development on Nav equipment, so "which sandbox do I use on Windows" is a question every Windows user will ask. The answer is WSL2, but nothing in the README says so — the platform badges list macOS and Linux and stop there.

What is missing

• A short "Windows" section in the README stating plainly: cplt does not run natively on Windows; WSL2 is the supported path.
• A step-by-step WSL2 setup guide covering the sequence users are currently reverse-engineering: wsl --install, then inside the distro install Node, gh, gh auth login, npm i -g @github/copilot, then cplt.
• An explicit warning that installing Copilot CLI on the Windows side does not work from WSL, because WSL inherits the Windows PATH and the npm shim cannot find node (see WSL2: Windows-side copilot binary on PATH fails with a misleading "runtime extraction failed" error #188).
• A note on where the project should live. Working out of /mnt/c/... is slow and has different filesystem semantics; the guide should say whether that is supported or whether the repo should live in the Linux filesystem.

Acceptance criteria

• README states the Windows/WSL2 position up front, near the platform badges
• A copy-pasteable WSL2 setup sequence exists, ordered so the Windows-side-install trap is avoided
• Guidance on project location (/mnt/c vs Linux filesystem)
• Linked from the Nav sandboxing guidance so Windows users find it without asking in Slack