I enabled it, entered my custom prompts successfully, then when I reloaded the page the option had disappeared entirely from my settings menu.
Having waded through the morass of buggy (including MacOS’s built-in expansion, which seems to only work around 20% of the time) and subscription-only services many times in the past, I thought to give the open-source Espanso another go - having found it flaky and unwieldy in the past - but its last commit was many months ago and I simply could not get it to recognise Ventura permissions.
It was then that I remembered that the excellent Dash (https://kapeli.com/dash), for which I had already paid a very reasonable one-off fee, has a snippets manager. And it’s perfect.