Feature Request: Expose a variable for the project directory root

Description

Hooks documentation, e.g., advises against using relative paths, so I have to traverse the whole directory tree, at least starting from HOME. This is (a) brittle, in case the directory is moved or renamed; (b) does not work across users within a team on the same project (unless they have the same directory structure.

I think moving/renaming a project directory probably breaks a lot of things, given the structure of .claude, but hooks are another example.

Desired Feature

Expose a variable/method/something so that I can refer to the PROJECT ROOT using an "absolute" path (i say "absolute" because "~" is sort of an alias but it is considered OK). This seems feasible given that it should refer to the "cwd" where claude is first invoked, which is in the context and displayed at the start of a session in the pretty header box.