CreateLayout: TarWriter on Unix loses unix file permissions, breaking aspire-managed exec bit

## Problem

After switching `tools/CreateLayout` to use `System.Formats.Tar.TarWriter` on macOS (see related issue), bundle extraction succeeded but the CLI then failed at runtime with:

```
Permission denied: /root/.aspire/bundle/managed/aspire-managed
```

`System.Formats.Tar.PaxTarEntry` defaults `Mode` to `0`, which extracts as `0000` (no permissions). Files like `aspire-managed` (and any other executable payload in the bundle) lose their executable bit.

## Impact

Even with the macOS PAX header fix in place, `aspire init` and any other operation that invokes `aspire-managed` from the extracted bundle would fail with a permission-denied error.

## Fix (applied)

Explicitly set `entry.Mode = File.GetUnixFileMode(filePath)` for non-Windows entries in `tools/CreateLayout/Program.cs::CreateArchiveAsync` so the source file's unix permissions are preserved in the tar archive.

Branch: `davidfowl/js-monorepo-dockerfile` — commit `c210d7f6d` (same commit as the PAX fix; both are required for bundle extraction to work end-to-end on macOS hosts).

## Notes

This is a general gotcha when using `System.Formats.Tar.TarWriter` on Unix hosts: the writer does not pick up source file modes by default. Any other place in the codebase that writes tar archives via `TarWriter` should be audited for the same omission.