GitHub sync round trip

Pin a schema that lives in a GitHub repository as this project's source of record. The first import lands it as a governed model; the source (owner, repository, branch, file path, last commit) stays pinned to the space, so when the file changes you get the governed second import: a merge preview of New / Modified / Removed nodes to review before the queued merge job runs (backup, parse, merge, database update - each step visible). Edits made in the model travel back the same way: the merge-export preview diffs the regenerated schema against the original file before anything is committed to the repository.