ci: skip buf breaking until main carries the proto module

buf breaking diffs against main#subdir=proto, but proto/ first lands in
this PR — the check can only fail before merge ('Module had no .proto
files'). Gate it on the base branch actually having protos, and disable
the action's PR comment which the job token lacks permission to post.

Author: lakhansamani

.github/workflows/ci.yml

@@ -38,6 +38,18 @@ jobs:
           # buf breaking needs main's history to diff against.
           fetch-depth: 0
 
+      # buf breaking can only diff against main once main actually carries
+      # the proto module; before this PR series merges, main has no proto/
+      # and `buf breaking` fails with "had no .proto files".
+      - name: Check base branch has protos
+        id: base
+        run: |
+          if git ls-tree -d origin/main proto | grep -q proto; then
+            echo "has_proto=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "has_proto=false" >> "$GITHUB_OUTPUT"
+          fi
+
       - uses: bufbuild/buf-action@v1
         with:
           input: proto
@@ -46,6 +58,11 @@ jobs:
           # failing the job on any unformatted .proto. Catches drift before
           # generated code can diverge.
           format: true
-          # Only run breaking on PRs (push to main has nothing to diff against).
-          breaking: ${{ github.event_name == 'pull_request' }}
+          # Only run breaking on PRs (push to main has nothing to diff
+          # against) and only once main carries the proto module.
+          breaking: ${{ github.event_name == 'pull_request' && steps.base.outputs.has_proto == 'true' }}
           breaking_against: 'https://github.com/${{ github.repository }}.git#branch=main,subdir=proto'
+          # The action's PR comment needs permissions the default
+          # GITHUB_TOKEN of this job lacks ("Resource not accessible by
+          # integration"); skip it.
+          pr_comment: false