cli: extension handling should not require leading dot in create-app-manifest.ts

Tracking issue for a TODO found in `packages/cli/src/lib/app/create-app-manifest.ts`:

```ts
// TODO: @jaysencpp, this is just 🫤, extensions should not require leading dot
```

Revisit the extension-handling logic so that extensions do not require a leading dot, per the original author's note.

Start in packages/cli/src/lib/app/create-app-manifest.ts and inspect the extension-handling logic near the noted TODO. Trace how extensions are read and applied, then update the behavior so an extension without a leading dot is accepted. Done means create-app-manifest no longer requires callers to provide the leading dot.

Written by the indexing model from the issue text.