Untrack the reference/ snapshot and gitignore it

reference/ is a read-only snapshot of the pre-migration machine, kept
locally for porting configs but not part of the flake. Stop tracking it
and ignore it so migrations don't churn the repo.

.gitignore

@@ -6,3 +6,4 @@
 Thumbs.db
 **/__pycache__
 .config/fish/conf.d/secrets.fish
+/reference/