When HERMES_ROOT was added for Nix-bundled TUI support, the fallback
was set to os.getcwd(). This overrode the TUI's own import.meta.dirname
resolution, so launching `hermes --tui` from outside the repo caused
the gateway client to look for venv/bin/python relative to the user's
working directory instead of the repo root.
Use PROJECT_ROOT (resolved from the source file location) as the
fallback, which is stable regardless of where the command is invoked.