Add a standalone DevTools app under `/packages/devtools`, and wire it into the existing build system with a dedicated `devtools` boolean flag.

[JerryWu1234]

Summary

Add a standalone DevTools app under /packages/devtools, and wire it into the existing build system with a dedicated devtools boolean flag.

Background

We want DevTools to live as its own package so it can be built independently and integrated cleanly with the current repo structure.

To support this:

• create the new package in /packages/devtools
• add a devtools boolean flag in /scripts/utils.ts
• add a corresponding builder in /scripts/build.ts

The build logic can follow the existing router build script pattern, and should simply run Vite for the DevTools package.

Tasks

• Create /packages/devtools
• Set up the package so it can be built with Vite
• Add a devtools boolean flag in /scripts/utils.ts
• Add a DevTools builder in /scripts/build.ts
• Reuse the router build script approach as reference
• Ensure the DevTools builder just runs Vite
• Verify the new build path works correctly in the repo build flow

Expected behavior

• The repo contains a standalone DevTools package at /packages/devtools
• The build system recognizes a devtools flag
• Running the DevTools build triggers a Vite build for /packages/devtools
• The implementation is consistent with the existing router build script structure

Notes

Please use the router build script as the implementation reference for how to register the builder and hook it into the current build pipeline.