Generate themes from wallpapers with Matugen

[qybaihe]

Summary

• add omarchy theme from-wallpaper with smart, dark, and light palette modes
• generate a schema-complete colors.toml from contrast-aware Material roles and wallpaper-harmonized ANSI colors
• copy the wallpaper into a normal user theme and optionally apply it through the existing theme pipeline
• install Matugen only on first use, protect existing themes behind --force, and replace them transactionally
• document the command and palette mapping

Verification

• focused theme-from-wallpaper shell test passes
• full test/cli passes
• test/shell: 218/222 files pass; three packaging checks require a separate omarchy-pkgs checkout, and the unrelated SSH retry timing test passed when rerun alone
• Matugen 4.2.0 integration tested with dark and near-white images; smart mode selected dark and light respectively
• generated dark theme applied in a live Omarchy VM; desktop, bar, notifications, menu, and wallpaper were visually checked before restoring the original state

Closes #8745