feat: add multi-checkout setup script

[NicholasSpisak]

Summary

• add multi-checkout helper script
• ignore .env.local symlinks

Testing

• not run (not requested)

[NicholasSpisak]

Testing my understanding of your SDLC - https://x.com/steipete/status/2011880333348913355

[steipete]

Thanks for sending this. I gave it a fresh maintainer pass against current main and tested the script in a temp checkout. It compiles and the dry-run path works, but this is broader than I want to carry in the canonical shared-scripts repo: it creates/removes whole checkout directories, writes a shared env template with project-specific secrets, and adds local workflow policy that is better kept in personal setup docs/scripts than shipped here. Closing to keep this repo focused on small portable helpers.