fix: append .exe suffix to Windows binary download filename (#418)

* fix: append .exe suffix to Windows binary download filename

The drone-ssh release publishes Windows assets with an .exe suffix
(e.g. drone-ssh-1.8.2-windows-amd64.exe), but entrypoint.sh built the
download filename without it, so every Windows runner failed with a 404
at the download step (ERR_DOWNLOAD_FAILED).

Append .exe when the detected platform is windows. This also keeps the
checksums.txt lookup working on Windows since entries match the exact
asset name. Add a windows-latest CI job that exercises the download,
checksum verification, and binary execution path.

Fixes #417

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix: detect Git Bash/MSYS/Cygwin uname output as windows platform

On windows-latest runners, bash steps run under Git Bash where
uname -s reports MINGW64_NT-10.0-<build>, so platform detection
rejected Windows runners with ERR_UNKNOWN_PLATFORM before the
download step was even reached. Map mingw*/msys*/cygwin* to windows.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/main.yml

@@ -793,6 +793,20 @@ jobs:
             echo "Output contains 'True'"
           fi
 
+  windows-binary-download:
+    runs-on: windows-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v7
+
+      - name: test binary download and checksum verification
+        shell: bash
+        run: |
+          export GITHUB_ACTION_PATH="$PWD"
+          export INPUT_CURL_INSECURE=false
+          export INPUT_CAPTURE_STDOUT=false
+          ./entrypoint.sh --help
+
   testing-script-error:
     runs-on: ubuntu-latest
     steps: