Restructure templates to include Nix development environment

Moved template directories into templates/ subdirectory (templates/vanilla/
and templates/nextjs/) with complete Nix configuration in each. This ensures
initialized projects have the full development environment with Nix, Bun,
and all tooling out of the box.

Changes:
- Created templates/vanilla/ with flake.nix, nix/, .envrc, and all files
- Created templates/nextjs/ with flake.nix, nix/, .envrc, and all files
- Updated template paths in nix/modules/template.nix

nix/modules/template.nix

@@ -9,13 +9,13 @@ in {
       };
 
       nextjs = {
-        description = "Next.js TypeScript template with App Router for Vercel";
+        description = "Next.js TypeScript template with App Router";
         path = "${root}/templates/nextjs";
       };
     };
 
     om.templates = {
-      javascript-vanilla = {
+      default = {
         template = templates.default;
         params = [
           {
@@ -26,7 +26,7 @@ in {
         ];
       };
 
-      javascript-nextjs = {
+      nextjs = {
         template = templates.nextjs;
         params = [
           {