ci: fix proto syntax check command — fixes #173

The proto validation infrastructure was added in commit e70ba44 but used
an invalid --syntax_check flag. Changed to use --descriptor_set_out=/dev/null
which validates syntax without generating files.

Proto validation flow (now complete):
1. go-test job: verify .proto files compile (syntax check) ✅
2. deploy-node job: validate protos match prod API responses ✅

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/deploy.yml

@@ -70,7 +70,7 @@ jobs:
           echo "Checking proto syntax..."
           for proto in proto/*.proto; do
             echo "  ✓ $(basename "$proto")"
-            protoc --proto_path=proto --syntax_check "$proto"
+            protoc --proto_path=proto --descriptor_set_out=/dev/null "$proto"
           done
           echo "✅ All .proto files are syntactically valid"