I want a to-do list app that syncs from a json file (or other human-readable data format), so that I can view and modify the file (via a CLI like jq) from my computer too, and it would still reflect on my phone when it syncs.
Does this exist? Preferably it uses a format simple enough that makes it possible / easy to modify it via jq.
Not exactly what you asked for but Joplin files are markdown. The app itself (very cross platform) renders them in a nice format. https://joplinapp.org/
A surprising number of people will tell you that the reason they prefer the closed-source Obsidian to Joplin is that Joplin doesn’t use Markdown files as its backend format to store its notes, but rather a database file. (They are formatted with Markdown, though.) I think the concerns they often express around lock-in are overblown, but this may mean it’s not what OP is looking for. I agree that the Joplin app is pretty nice, though more polished and featureful on desktop.
You can literally export all your notes in 2 clicks. You right-click on your notebook, hover over the export menu and select Markdown. Choose a location and Joplin will create a folder with all your notes in Markdown format inside.