Add ~/.local/bin, ~/.cargo/bin, ~/go/bin, ~/.npm-global/bin to the
systemd unit PATH so tools installed via uv/pipx/cargo/go are
discoverable by MCP servers and terminal commands.
Uses a _build_user_local_paths() helper that checks exists() before
adding, and correctly resolves home dir for both user and system
service types.
Co-authored-by: Kal Sze <ksze@users.noreply.github.com>