Relocate SSH inventory and rbw socket wiring from `modules/lib/` to
dedicated `modules/ssh/` modules to improve logical grouping.
- Move `ssh-inventory.nix` to `modules/ssh/inventory.nix`.
- Move `rbw-ssh-socket.nix` to `modules/ssh/rbw.nix`.
- Relocate `users-merge.nix` logic into `modules/system/users.nix` to
reduce dependency on generic library modules.
- Update `docs/conventions.md` to reflect new module paths.
- Refactor `modules/deploy/navi.nix` to use a local helper for
generating Navi hive configurations.
- Add Pi5 specific Niri KDL configuration snippets.
Olivier