Add GPLv3 LICENSE and update CI golangci-lint step

Add full GNU General Public License v3 (LICENSE) to the repo. Update .github/workflows/ci.yml to install golangci-lint via `go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.1.6` and run the linter directly (`golangci-lint run`) instead of using the golangci/golangci-lint-action.

Author: youhide

.github/workflows/ci.yml

@@ -43,7 +43,8 @@ jobs:
         with:
           go-version: "1.25"
 
+      - name: Install golangci-lint
+        run: go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@v2.1.6
+
       - name: golangci-lint
-        uses: golangci/golangci-lint-action@v7
-        with:
-          version: v2.1.6
+        run: golangci-lint run