Overwrite Mode

add an overwrite mode flag to the `generate` commands `--out` flag so that we can have appending be the default behaviour of the command when outputting to a file and can switch to overwriting when the flag is on.

Start by locating the generate commands and their --out flag handling in the Go CLI, then inspect how file output currently behaves. Done means output appends by default and a new overwrite mode flag switches the same commands to overwriting the file; add or run relevant command tests if the project provides them.

索引モデルが issue の本文から書いたものです。