Extract MarkdownParser.cs from MainWindow.axaml.cs

[mock1ngbb]

STATIC UTILITY — no state dependencies, pure function extraction.

Extract from MainWindow.axaml.cs lines ~5364-5946:

• ParseMarkdown() → static MarkdownParser.ParseMarkdown()
• ParseInlineMarkdown() → static MarkdownParser.ParseInlineMarkdown()
• FlushListItems() → static MarkdownParser.FlushListItems()
• MarkdownBlock class → nested in MarkdownParser

New file: Services/MarkdownParser.cs in GithubLauncher.Services namespace

Verification: dotnet build passes with 0 errors.
Doable by: haiku/cline-agent under 30 min.