refactor(ssh): restructure inventory and modularize helper logic

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.

modules/ssh/nixos/default.nix

@@ -36,7 +36,7 @@
         {
           assertions = [{
             assertion = cfg.enable -> missingKeys == [ ];
-            message = "chiasson.ssh.inbound: users enabled but missing public key in `modules/lib/ssh-inventory.nix`: ${lib.concatStringsSep ", " missingKeys}";
+            message = "chiasson.ssh.inbound: users enabled but missing public key in `modules/ssh/inventory.nix`: ${lib.concatStringsSep ", " missingKeys}";
           }];
         }
         (lib.mkIf cfg.enable {