sg: Migrations are embedded in the binary, making developing new migrations painful

Created by: camdencheek

Migrations are currently embedded in the binary using //go:embed, which is problematic when developing migrations because, in order to use sg migrate, sg needs rebuilt every time a migration changes, so I have to run ./dev/sg/install.sh after every migration iteration.

Ideally, if we're in-repo, sg just uses the migrations on the filesystem, allowing live edits to a migration file.