Trying to apply the idea of POSSE (Publish on your Own Site, Syndicate Elsewhere) to my own posting. Here I'll summarise my results.
Automatically tweet public links I add to Shaarli (this would be a very convenient solution). But did not work because Twitter now requires you to jump through hoops to get access to the Twitter API from within your own Twitter account! Most egregiously, there is no way to complete their application procedure without handing over and verifying your phone number. Not doing that, Twitter can forget about it.
I still liked the idea of using Shaarli as my canonical source, so to speak, so let's look into Shaarli -> Mastodon instead. Alright, that's awesome and works great (well, unfortunately my Mastodon instance does not render Markdown, but that's not shaarli2mastodon's fault - I'll just have to avoid using Markdown in Shaarli notes posted to Mastodon).
With Shaarli -> Mastodon working, let's explore whether Lond's Mastodon <-> Twitter cross-poster can tweet our toots.
And yes, indeed, that worked right away. All I had to do was authorize the app on both my Mastodon and Twitter accounts. It needs lot of permissions, but hey, at least its source code is public, and it's even possible to self-host it.
That's positively awesome! I can now select to post any Shaarli note to Mastodon, and doing so will automatically
cross-post the same to Twitter too. Well done, POSSE!