CNA: add .github to allow list when checking if directory is empty

### Link to the code that reproduces this issue https://github.com/chrisweb/reproduction_create-next-app_empty_directory ### To Reproduce when using CNA and there is a .github directory already present then CNA exists ### Current vs. Expected behavior have CNA ignore the .github directory because it is on the allow list of the "check if is empty" function ### Provide environment information ```bash Operating System: Platform: win32 Arch: x64 Version: Windows 10 Home Available memory (MB): 32681 Available CPU cores: 16 Binaries: Node: 22.13.0 npm: 11.4.0 Yarn: N/A pnpm: 9.15.3 Relevant Packages: next: 15.3.2 // Latest available version is detected (15.3.2). eslint-config-next: 15.3.2 react: 19.1.0 react-dom: 19.1.0 typescript: 5.8.3 Next.js Config: output: N/A ``` ### Which area(s) are affected? (Select all that apply) create-next-app ### Which stage(s) are affected? (Select all that apply) next dev (local) ### Additional context use case: after creating a new directory, you add a prompt template for github copilot into your .github/prompts directory, this prompt instructs github copilot to install next.js using a CNA command