install: report unsupported operating systems

[1fanwang]

Why

On FreeBSD, install.sh reports Windows detected but winget not found because every operating system other than macOS and Linux falls into the Windows branch. Copilot CLI does not publish a FreeBSD binary, so the installer should report the platform as unsupported.

What changed

The installer now treats Cygwin, MinGW, and MSYS as Windows. Other unknown systems fail with their actual uname -s value.

References

#3710

Testing

Scenario	Result
FreeBSD against upstream/main	Reproduced the incorrect Windows message
FreeBSD with this change	Reports Unsupported operating system FreeBSD
MinGW	Still invokes winget install GitHub.Copilot
macOS arm64 and Linux x64	Still select their existing release assets

Raw logs

$ tmp=$(mktemp -d)
$ printf '#!/bin/sh\ncase "$1" in -m) echo amd64;; *) echo FreeBSD;; esac\n' > "$tmp/uname"
$ chmod +x "$tmp/uname"
$ git show upstream/main:install.sh > "$tmp/before.sh"
$ PATH="$tmp:/usr/bin:/bin" bash "$tmp/before.sh"; echo "exit=$?"
Installing GitHub Copilot CLI...
Error: Windows detected but winget not found. Please see https://gh.io/install-copilot-readme
exit=1

$ PATH="$tmp:/usr/bin:/bin" bash install.sh; echo "exit=$?"
Installing GitHub Copilot CLI...
Error: Unsupported operating system FreeBSD
exit=1

bash -n install.sh and git diff --check pass.

[Copilot]

🟢 Approval recommended

The focused detection change correctly handles FreeBSD while preserving supported platform behavior.

Pull request overview

Updates OS detection so unsupported systems no longer fall through to Windows installation logic.

Changes:

• Recognizes Cygwin, MinGW, and MSYS as Windows.
• Reports unknown platforms using their uname -s value.

File summaries

File	Description
install.sh	Adds explicit Windows matching and unsupported-OS handling.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 0
• Review effort level: Balanced

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[1fanwang]

Superseded by #4762, which landed the same detection change. Closing.