Deploy labeled pull requests to permanent staging

[Perdolique]

Summary

Add an explicit pull request label that deploys the labeled PR to the permanent staging Worker at staging.metsik.app.

Current behavior

Pull requests run deploy:versions:staging, which uploads an isolated Cloudflare preview version. Updating the permanent staging Worker still requires a manual pnpm run deploy:staging deployment.

Desired behavior

• Keep the existing preview deployment as the default for pull requests
• Introduce a deploy:staging pull request label
• When the label is added, deploy that pull request's current HEAD with pnpm run deploy:staging
• Update the permanent metsik-staging Worker served at staging.metsik.app
• Expose the label-triggered deployment as a visible GitHub Actions run

Acceptance criteria

• An open pull request targeting master can be deployed to permanent staging by adding the deploy:staging label
• Pull requests without the label continue to use preview version uploads only
• Production deployment behavior remains unchanged