fix: Prevent concurrent deployments by excluding main branch from check workflow

Root cause: Both auto-update and check workflows were triggered simultaneously
on push to main branch, causing a race condition when deploying to the same
CloudFormation stack. This resulted in deployment failures with ECR permission
errors.

The error occurred because:
1. PR merge triggered both workflows at 2025-11-13T01:24:08Z
2. Both tried to deploy to docker-selenium-lambda-prod stack concurrently
3. CloudFormation CREATE_FAILED on DemoLambdaFunction due to conflict
4. Stack rolled back to UPDATE_ROLLBACK_COMPLETE state

Solution: Exclude main branch from check workflow since auto-update already
handles main branch deployments. Added workflow_dispatch for manual testing.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: umihico

.github/workflows/check.yml

@@ -1,7 +1,10 @@
 name: check
 
 on:
-  - push
+  push:
+    branches-ignore:
+      - main
+  workflow_dispatch:
 
 permissions:
   id-token: write