Configure nix.gc to automatically clean up old home-manager generations
weekly, which complements the existing system-level gc. Uses
--delete-older-than 10d on Linux to maintain a rollback window, but
overrides to -d on Darwin to work around a launchd bug where multi-part
options aren't properly split into separate arguments.
Create base-linux and base-darwin modules to cleanly separate platform-
specific role imports from shared roles. This prevents importing modules
that require platform-specific home-manager modules (like plasma-manager
on NixOS) in environments where they don't exist (like nix-darwin).
- base-linux includes: plasma-manager, i3+sway
- base-darwin includes: aerospace
- roles/default.nix now only contains truly cross-platform roles
This architecture makes it immediately clear which roles are shared
versus platform-specific and makes it easy to add new platform-specific
roles in the future.
