Symbol to signify the root of a project

If you are looking for a convention for use in communicating with a team, I'd suggest the project name followed by a /. This makes it clear as to what project you are referring to. If the project name is already implied by the context, it seems to be the convention to simply use a subdirectory name, with or without a trailing slash. See here and here for examples from Linux-kernel related documentation.

I'm not aware of any such convention. In Autoconf, variables top_srcdir and abs_top_srcdir points to the root of a project. In git, this does the job:

git rev-parse --show-toplevel

However, if you are looking for a single character symbol, I suggest borrowing the tee character: ⊤ (U+22A4, &#8868). I don't think it has ever been used for that, but it captures the idea of top.

the root of a project

What means the root of the project exactly ? Given which context ? Which types of projects ? Are you talking about a deployed web projects ? A source tree of a web projects ? A command line utility written in C ? Or in Java ? Or Go ?

Each language and framework provides its on sets of predefined structures to follow. The root of the project is then, either the root of the vcs, which may store many assets not strictly related to the business of the software, or the root according to the given framework / language you are working with, in which case, i assume it is safe to say, it can be anything because they are so many different fw for so many different concerns.