GTK/desktop lane: add workstation mac-defaults validation check

[mdheller]

Closed by PR #160.

Resolution:

• Added profiles/linux-dev/workstation-v0/bin/check-mac-defaults.sh.
• Added .github/workflows/workstation-mac-defaults.yml.
• Documented the read-only validation hook in profiles/linux-dev/workstation-v0/gnome/README.md.
• Preserved the issue boundaries: no active keybindings, GNOME defaults behavior, package manifests, sourceos, or doctor.sh changes.

Validation evidence is recorded in PR #160.