refactor(core): name the GUI and CLI executables once

`is_openlogi_foreground_id` carried four literal `.exe` names, two of which were just `Helper::executable()` with a suffix, while the GUI's and the CLI's names had no owner at all: xtask spelled them again in the Linux package list and three times in the macOS bundle embed.

`brand::GUI_EXECUTABLE` and `brand::CLI_EXECUTABLE` own them; the foreground check matches a Windows path's file stem against the four executables instead of four spelled-out file names, and xtask's bundle contents read brand. The cargo `--package`/`--bin` arguments stay literal: those are crate names that happen to coincide, which is also why no guard is added here.