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.

justfile

@@ -0,0 +1,8 @@
+# Repo helpers. Run `just` to list recipes.
+
+default:
+    @just --list
+
+# Sync live DMS config into this host's seed dir. Pass --dry-run, --settings-only, or a host module name.
+sync-dms *args='':
+    ./modules/desktop/shells/dms/_private/sync-seed.sh {{args}}