feat: add signtool to plugin repository

- Migrate signing tool from GoTunnel main project
- Self-contained, no external dependencies
- Updated CI workflow to build locally

.github/workflows/sign.yml

@@ -20,10 +20,7 @@ jobs:
           go-version: '1.21'
 
       - name: Build signtool
-        run: |
-          git clone --depth 1 https://github.com/your-org/gotunnel.git /tmp/gotunnel
-          cd /tmp/gotunnel
-          go build -o /usr/local/bin/signtool ./cmd/signtool
+        run: go build -o signtool ./tools/signtool
 
       - name: Sign plugins
         env: