fix(ci): add GOPROXY and GOSUMDB env vars for consistent downloads

Ensures modules are downloaded from official proxy with proper checksum
verification to avoid checksum mismatch errors in CI.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -22,6 +22,9 @@ jobs:
 
       - name: Download dependencies
         run: go mod download
+        env:
+          GOPROXY: https://proxy.golang.org,direct
+          GOSUMDB: sum.golang.org
 
       - name: Build
         run: go build ./...