That looks very interesting. An essential feature for me is data, including my notes and annotations, that will remain usable in the long run - many decades into the future.
Otherwise, my work of today is lost. What happens if you stop making Cahier someday or if I need to move to another system?
I agree with you. We're expanding our support for import/export as time goes by, there will be no lock-in. We already have markdown support and plan on adding markdown import/.bib export and import/pdf export. Plenty of formats to choose from. Your notes can already be exported and opened on Obsidian today.
If there's enough interest, I'm also open to documenting the file format we use.
Otherwise, my work of today is lost. What happens if you stop making Cahier someday or if I need to move to another system?