fix(build): say so when the ISO ships without archsetupHEADmain

The 2026-08-02 build ran without ARCHSETUP_DIR set and produced a bare ISO with no complaint. A recovery image missing its setup tooling only reveals that on the machine you're recovering, which is the worst place to find out.

The build now warns when ARCHSETUP_DIR is unset or points at a non-directory, and the closing summary reports baked or not baked either way. Both go to the build log, since mkarchiso's output buries anything printed mid-run.

The remediation hint uses sudo's VAR=value form because build.sh needs root and env_reset drops an exported variable. It uses $HOME rather than a tilde, which zsh leaves unexpanded in an assignment.

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