Add CI workflow and address PR feedback

- Create `.github/workflows/ci.yaml` for PR checks on main.
- Remove vendor directory and related workflow step per PR feedback.
- Update CI workflow to use `git status --porcelain` for uncommitted files check.
- Ensure codebase is formatted and `go.sum` is updated.

Author: google-labs-jules[bot]

.github/workflows/ci.yaml

@@ -22,13 +22,13 @@ jobs:
       - name: Run go mod tidy
         run: go mod tidy
 
-      - name: Run go mod vendor
-        run: go mod vendor
-
       - name: Run gofumpt against code
         run: gofumpt -l -w .
 
       - name: Ensure no modified or uncommitted files
         run: |
-          git add .
-          git diff --staged --exit-code
+          if [ -n "$(git status --porcelain)" ]; then
+            echo "Modified or uncommitted files found"
+            git status --porcelain
+            exit 1
+          fi