refactor: centralize module options, remove osConfig pattern
- Create modules/options/{desktop,dev}.nix with shared option declarations
- Remove duplicated options from desktop/{home,nixos}.nix and dev/{home,nixos}.nix
- Replace osConfig bridging with home-manager.sharedModules propagation
- Fix hardcoded username in core.nix: use mkDefault instead of default value
- Add code task files for planned refactoring work