ci(workflow): add concurrency blocks to prevent duplicate runs (#135)

Add concurrency control to all review workflows:
- opencode-review.yml: Cancel in-progress for same PR
- claude-code-review.yml: Cancel in-progress for same PR
- claude.yml: Cancel in-progress for same issue/PR

This prevents duplicate reviews when PRs are updated rapidly
or multiple comments are posted in quick succession.

Co-authored-by: Test User <test@example.com>

.github/workflows/opencode-review.yml

@@ -11,6 +11,11 @@ on:
       - ".gitignore"
       - "pyproject.toml"
 
+# Cancel in-progress runs for the same PR to avoid duplicate reviews
+concurrency:
+  group: opencode-review-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
 jobs:
   opencode-review:
     runs-on: ubuntu-latest