docs: add native Windows installation instructions to README

[johnmungandiall]

Add PowerShell installation instructions for native Windows support.

[johnmungandiall]

[alt-glitch]

Likely duplicate of #8806 — same docs gap (README says native Windows not supported but install.ps1 exists). Competing PRs: #8821, #10565, #15461.

[teknium1]

Thanks for this — appreciate the work. We're closing the entire cluster of open native-Windows PRs (44 of them spanning installer, terminal routing, file ops, gateway PID handling, encoding, docs, and more) because the surface area needs a designed, consolidated approach rather than piecemeal merges. Cherry-picking individual fixes keeps leaving inconsistencies and we'd rather land Windows support properly, in one coherent pass.\n\nYour PR is catalogued in our internal Windows support plan. When we pick this back up (soon), we'll mine every PR in the cluster for its fix shape and credit all contributors whose work informs the final patch via lines. Watch for the consolidating PR and feel free to chime in with context on the specific failure mode you were hitting.\n\nClosing for now, not as a rejection of the fix — just queueing it for the designed rollout. Thanks again.