It's in a very MVP-state, but I created stick, "shareable Git-powered notebooks". It meets some (but not all) of your criteria. It is FOSS (MIT). Features:

It's in a very MVP-state, but I created stick, "shareable Git-powered notebooks". It meets some (but not all) of your criteria. It is FOSS (MIT). Features: - Notebooks are standard git repositories containing .md files. - Notebooks may be used privately, shared with one or more users privately, or shared publicly. - Notes may be customized to improve usability, such as rendering a to-do list with automatic de-duplication and sorting. Repository: https://code.rocketnine.space/tslocum/stick https://code.rocketnine.space/tslocum/stick Demo (read-only except checkboxes): https://stick.rocketnine.space/#login/c89f5381659ad34bd84967fdbbb5e76834495063dc93ed3902bf4e186f4def90 https://stick.rocketnine.space/#login/c89f5381659ad34bd84967...