Add copied-prompt guidance dialog to setup wizard

[Copilot]

The setup wizard needs stronger next-step guidance after copying the generated prompt. Users should be told to create a repository and paste the prompt into the starter text box.

• Copy confirmation

  • Opens a modal dialog after the prompt is successfully copied.
  • Keeps the existing inline status message, updated with the repository/starter-text-box instruction.

• Dialog behavior

  • Uses the native <dialog> modal API.
  • Supports explicit close, backdrop click, and Escape dismissal.
  • Resets the copied button state when the dialog closes.

• Accessibility and styling

  • Labels the dialog from its visible title with aria-labelledby.
  • Adds wizard-consistent modal, backdrop, and action button styling.