fix(clay): clear error for native Windows instead of generic OS failure

[GirishVerm]

Git Bash/MSYS/Cygwin report uname -s as MINGW64_NT-, MSYS_NT-, or CYGWIN_NT-*, which fell through to the generic "unsupported OS" error. No Windows CLI binary is published yet, so point users at WSL instead.

[GirishVerm]

Context for this PR: it addresses #9 (native Windows support for the clay launcher).

Scope: checksums.txt only lists clay-darwin-arm64, clay-darwin-x64, clay-linux-arm64, and clay-linux-x64 — there's no Windows CLI binary published, and building one is a change to the clay CLI's own build/release pipeline, not something this plugin repo controls. So this PR doesn't add Windows support; it addresses the "at minimum a clear error message" fallback the issue also asked for.

Change: bin/clay's OS detection now recognizes Git Bash/MSYS/Cygwin's uname -s output (MINGW64_NT-*, MSYS_NT-*, CYGWIN_NT-*), which previously fell through to a generic unsupported OS: $os error. It now fails fast with a message pointing users at WSL as the supported path, still using the same validation_error category/exit code as the existing OS/arch checks.

Tested by shadowing uname -s to report MINGW64_NT-10.0-26200 — confirms the new branch fires with exit code 2 — and confirmed the normal macOS path still resolves and runs (clay --version) unaffected.

Issue #9 stays open to track actual native Windows binary support as the larger ask.