Fix System.CommandLine beta5 breaking changes

[Copilot]

System.CommandLine 2.0.0-beta5 introduced breaking API changes that fail compilation. Updates Program.cs to conform to the new API.

Breaking changes addressed

• Option<T> constructor — description parameter removed; set via Description property instead
• IsRequired → Required
• AddOption() → Add()
• SetHandler() → SetAction(ParseResult, CancellationToken) — option values retrieved explicitly via parseResult.GetValue(option) rather than positional binding
• command.InvokeAsync(args) → command.Parse(args).InvokeAsync()

Example migration

// Before (beta4)
var opt = new Option<string>("--output-file", "Path to output.")
    { IsRequired = true };
rootCommand.AddOption(opt);
rootCommand.SetHandler((output) => DoWork(output), opt);
return await rootCommand.InvokeAsync(args);

// After (beta5)
var opt = new Option<string>("--output-file")
    { Description = "Path to output.", Required = true };
rootCommand.Add(opt);
rootCommand.SetAction(async (ParseResult pr, CancellationToken ct) =>
{
    var output = pr.GetValue(opt)!; // Required = true guarantees non-null
    return await DoWork(output);
});
return await rootCommand.Parse(args).InvokeAsync();

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