Rewrite the main project docs to present SHIFT as the product and Plasma Mobile as the upstream base. Keep compatibility-sensitive runtime identifiers such as org.kde.plasma.mobileshell and ~/.config/plasma-mobile explicit in the hacking guide instead of treating them as product identity.
SHIFT now ships its own wallpaper package; remove
breeze6-wallpapers and plasma6-workspace-wallpapers from the
runtime dependency list in HACKING.md. Add .preview-data/ to
.gitignore.
