feat(lobtop): add cmux module with declarative ghostty and cmux config

[stackptr]

Summary

• Adds modules/darwin/programs/cmux.nix with a programs.cmux.enable option that installs cmux via Homebrew
• Declaratively manages ~/.config/cmux/cmux.json via programs.cmux.settings (JSON format); $schema and schemaVersion are injected by the module automatically
• Declaratively manages ~/.config/ghostty/config via programs.cmux.ghostty.settings and programs.cmux.ghostty.palette
• Enables the module in hosts/lobtop/programs.nix with settings matching the existing hand-written config files

Test plan

• Run nh darwin switch .#lobtop and verify cmux is installed
• Verify ~/.config/cmux/cmux.json is generated with correct content including $schema and schemaVersion
• Verify ~/.config/ghostty/config is generated with correct colors, font, and all 16 palette entries

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