Installer rejects windows-arm64 despite publishing that binary

Description

The installer rejects Windows ARM64 with Unsupported OS/Arch: windows/arm64, even though opencode-windows-arm64.zip is built, signed and published on every release.

The arch allowlist in install omits windows-arm64:

linux-x64|linux-arm64|darwin-x64|darwin-arm64|windows-x64)

The detection just above it already maps MINGW*/MSYS*/CYGWIN* to windows and aarch64 to arm64, so the correct combo is computed and then rejected by the allowlist.

Windows ARM64 is otherwise a first-class target: it is declared in packages/opencode/script/build.ts, and publish.yml signs it, verifies the signature, repacks it and uploads opencode-windows-arm64.zip.

This also blocks opencode upgrade for curl installs, since the upgrade path re-runs this same script.

Steps to reproduce

1. On Windows ARM64, open Git Bash
2. Run curl -fsSL https://opencode.ai/install | bash
3. It exits with Unsupported OS/Arch: windows/arm64

I don't have ARM64 hardware, so I confirmed it from the script instead: the allowlist rejects windows-arm64, and the asset it would download returns HTTP 200 on the latest release.

Operating System

Windows 11 ARM64

Terminal

Git Bash