Fix dev server spawn on Windows

[cpjet64]

Summary

• run the local tsx CLI through the current Node executable instead of spawning bare npx
• avoid Windows ENOENT failures when npm run dev starts the watcher
• add a regression test for the dev-server command used by the watcher

Verification

• node scripts/dev-server.test.mjs
• npm test
• npm run typecheck
• npm run build
• npm run dev smoke test with local DEVSPACE_* env; server reached http://127.0.0.1:7676/mcp before the intentional timeout