Fix shebang recommendation in file-based apps docs

[Copilot]

#!/usr/bin/env dotnet causes dotnet to silently consume CLI arguments that match its own parameters (e.g., --help) instead of forwarding them to the app.

Changes

• docs/core/sdk/file-based-apps.md — Shell execution section
  • Updated recommended shebang from #!/usr/bin/env dotnet to #!/usr/bin/env -S dotnet --
    • -S: tells env to split the remaining text so -- is a distinct argument
    • --: tells dotnet to forward all subsequent arguments to the app
  • Added explanatory paragraph describing why the new form is needed
  • Added note recommending #!/usr/bin/env dotnet as a fallback for systems where -S isn't supported, with a caveat about argument forwarding

#!/usr/bin/env -S dotnet --
#:package Spectre.Console

using Spectre.Console;

AnsiConsole.MarkupLine("[green]Hello, World![/]");