docs: add usage examples to --help output (closes #374)

[cemheren]

What

Appends an Examples block to PrintHelp() so --help shows common workflows at a glance, matching the convention of git --help / curl --help.

Examples shown

Examples:
  ExcelConsole data.csv                              Launch desktop wallpaper with a CSV
  ExcelConsole data.csv --export-md out.md           Convert CSV to a Markdown table
  ExcelConsole data.csv --export-html out.html       Convert CSV to a styled HTML table
  ExcelConsole data.csv --export-json -              Print CSV as JSON to stdout
  ExcelConsole --list-extensions                     List installed extensions

Notes

• Uses only flags that actually exist today (the issue's sample referenced --info and -d, which aren't implemented yet — omitted to keep help truthful).
• Pure string output in one method, no logic changes.
• dotnet build clean (0 warnings, 0 errors); verified output via --help.

Closes #374