Agendafs is a FUSE-based filesystem for writing notes. It creates a shim over vdir storages, allowing you to write notes as regular files and synchronize them back to a caldav server. It supports directories, categories, arbitrary metadata and access rights.
Agendafs is still in early development and is alpha-level software. It probably leaks memory, destroys files and might summon demons. However, I use it daily for my note-taking mostly without issues. If you try it out, I recommend creating a new calendar specifically for agendafs and also regularly backing up your notes elsewhere.
Prerequisites: scdoc (man pages), C compiler (currently only tested with gcc), libfuse, libuuid, libical and pkg-config. Optionally a shell.nix exists which sets up all dependencies for you.
Some fields, such as categories, are accessed via xattributes (xattr(7)), for that you will need to have the xattribute utilities installed.