Fix template metadata and naming

- Changed omnix template name from "default" to "vanilla" for clarity
- Updated flake descriptions to describe project type, not template repo
- Added {{package-name}} variable to vanilla/package.json for substitution

Ensures initialized projects have correct metadata and users see "vanilla"
and "nextjs" as template options during initialization.

nix/modules/template.nix

@@ -15,7 +15,7 @@ in {
     };
 
     om.templates = {
-      default = {
+      vanilla = {
         template = templates.default;
         params = [
           {