refactor: replace duplicate bin/dev with symlink to generator template

The spec/dummy/bin/dev file was identical to lib/generators/react_on_rails/bin/dev.
Made the generator template the canonical version and replaced the dummy app's
copy with a symlink to avoid maintaining duplicate code.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: justin808

spec/dummy/bin/dev

@@ -0,0 +1 @@
+../../../lib/generators/react_on_rails/bin/dev