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/desktop/niri/pi5.nix

@@ -0,0 +1,39 @@
+# Pi5 + DSI DRM KDL snippets (`desktop.niri.raspberryPi5DrmWorkaround`) — lives in `flake.lib`.
+{ ... }:
+let
+  drmExtraConfig = ''
+    debug {
+      render-drm-device "/dev/dri/renderD128"
+      ignore-drm-device "/dev/dri/card1"
+      ignore-drm-device "/dev/dri/card2"
+    }
+  '';
+in
+{
+  flake.lib.pi5NiriKdl = {
+    inherit drmExtraConfig;
+
+    # Keep in sync with DMS greeter niri template when upstream edits it.
+    dankGreeterCompositorConfig = ''
+      hotkey-overlay {
+          skip-at-startup
+      }
+
+      environment {
+          DMS_RUN_GREETER "1"
+      }
+
+      ${drmExtraConfig}
+
+      gestures {
+         hot-corners {
+           off
+         }
+      }
+
+      layout {
+        background-color "#000000"
+      }
+    '';
+  };
+}