> any tips, guides, prayers, incantations for how to get changes in one to not clobber the other

One caveman way:

1. Start your project using Xcode, use it to commit to GitHub, GitLab, whereever. In terminal, change into the dir that has the .git in it and launch claude.

2. Teach Claude Code your own system's path and preferred simulator for build testing. From then on it will build-test every change, so teach it to only commit after build passes. (By teach, I mean, just tell it, then from time to time, tell it to propose updates to claude.md in your project.)

3. Make sure before a PR or push that the project still builds in Xcode, if it doesn't, you can eyeball the changes in Xcode's staged changes viewer and undo them. If you change files via IDE, when you're back in Claude just say: I changed [specific/file, or "a lot of files"].

No xproj or symlinks get harmed in the making of your .swifts.