Add DMS configuration and helper scripts

- Introduced a new `justfile` for managing DMS-related tasks, including a recipe for syncing live DMS configuration.
- Added a `devshell.nix` file to set up a development environment with necessary packages and shell hooks for DMS.
- Updated DMS settings to replace `defaultSettingsFile` with `defaultSeedDir` for improved configuration management.
- Created a new script `sync-seed.sh` to facilitate syncing DMS settings from the live configuration to host-specific directories.
- Enhanced documentation and examples for DMS configuration and usage.

modules/devshell.nix

@@ -0,0 +1,31 @@
+{ inputs, lib, ... }:
+{
+  perSystem =
+    {
+      pkgs,
+      system,
+      ...
+    }:
+    lib.optionalAttrs (lib.elem system [
+      "x86_64-linux"
+      "aarch64-linux"
+    ]) {
+      devShells.default = pkgs.mkShell {
+        packages = [
+          inputs.navi.packages.${system}.default
+          pkgs.just
+          pkgs.jq
+        ];
+        shellHook = ''
+          echo "Repo helpers (from repo root):"
+          echo "  just                       # list justfile recipes"
+          echo "  just sync-dms [--dry-run]     # copy live DMS config into host seed dir"
+          echo ""
+          echo "Navi fleet deploy:"
+          echo "  navi apply --on <host>     # build + switch one host"
+          echo "  navi apply-local --node 14900k --sudo   # switch locally (needs root)"
+          echo "  navi tui                   # interactive fleet dashboard"
+        '';
+      };
+    };
+}