Refactor UpgradeAssistant to use CommandLineParser

[Copilot]

Replaces custom argument parsing in tools/LinkDotNet.Blog.UpgradeAssistant/Program.cs with CommandLineParser library and adopts centralized package versioning via Directory.Packages.props.

Changes

• CommandLineOptions.cs (new): Defines command-line options using CommandLineParser attributes
• Program.cs:
  • Uses Parser.Default.ParseArguments<CommandLineOptions>() instead of custom parsing logic
  • Removes 27 lines of manual argument handling
  • Maintains custom help/version display using Spectre.Console (consistent with CriticalCSS tool pattern)
• LinkDotNet.Blog.UpgradeAssistant.csproj: Adds CommandLineParser package reference (version managed centrally)

Before/After

// Before: Manual parsing with switch statement
static void ParseCommandLineArguments(string[] args, ref string targetPath, ...)
{
    var i = 0;
    while (i < args.Length) {
        switch (args[i]) {
            case "-p" or "--path" when i + 1 < args.Length:
                i++; targetPath = args[i]; break;
            // ...
        }
    }
}

// After: Declarative options with CommandLineParser
public class CommandLineOptions
{
    [Option('p', "path", Required = false, Default = ".")]
    public string TargetPath { get; init; } = ".";
    // ...
}

return await Parser.Default.ParseArguments<CommandLineOptions>(args)
    .MapResult(async opts => await RunWithOptions(opts), _ => Task.FromResult(1));

Original prompt

For tools/LinkDotNet.Blog.UpgradeAssistant/Program.cs use CommandLineParser to parse the arguments rather than the custom made solution. Adopt the Directory.Packages.props as well

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