ci: protect synced folder from manual changes

[vgpopov]

Summary

• Adds a GitHub Actions workflow that blocks PRs modifying charts/platforma/ unless the author is github-actions[bot] (the sync bot)
• Ensures the synced folder stays consistent with the source repo and cannot be accidentally changed by humans

How it works

• Triggers on PRs targeting main
• Skips the check for github-actions[bot] actor
• Fetches changed files via GitHub API, fails if any are under the protected folder

Test plan

• Open a test PR that modifies a file in charts/platforma/ — should fail
• Verify the sync bot can still push directly to main without issues
• Enable branch protection rule requiring this check to pass

[notion-workspace]

[backend/k8s] Move platforma-helm into pl and setup auto sync

[mike-ainsel]

Good to go