alloy migrate

Rewrites alloy.toml in the current folder as .config.aly, then removes alloy.toml. While both files exist, alloy.toml wins.

The new file keeps every key and every comment, and the [fmt] table of the project sets its quotes and its indent. Before it writes, the command loads the new file and compares the two configs. When they differ, it writes nothing. It never overwrites a .config.aly.