fix(release): add test step, fix checkout version, improve brew formula

- Fix actions/checkout@v6 → v4 (v6 doesn't exist)
- Add Go test step before GoReleaser to prevent shipping broken binaries
- Add license and test block to Homebrew formula

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/release.yml

@@ -13,7 +13,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v6
+        uses: actions/checkout@v4
         with:
           fetch-depth: 0
 
@@ -23,6 +23,9 @@ jobs:
           go-version-file: server/go.mod
           cache-dependency-path: server/go.sum
 
+      - name: Run tests
+        run: cd server && go test ./...
+
       - name: Run GoReleaser
         uses: goreleaser/goreleaser-action@v6
         with: