Strengthen Step 0 welcome scaffolding and learner actions

[Copilot]

workshop/00-welcome.md was underperforming on active learning and scaffolding: the welcome step mostly asked learners to read and self-assess, and it lacked a lightweight orientation section before path selection. This updates Step 0 so learners do a small amount of concrete setup-thinking up front and leave the page knowing exactly where to go next.

• Add upfront scaffolding

  • Introduce a Before You Start section immediately after the opening paragraph.
  • Keep the scope intentionally small: account + browser only, without pulling Step 1 setup work into Step 0.

• Convert passive self-check into an active warm-up

  • Replace the recognition-only checklist with concrete actions:
    • open account/billing settings
    • identify plan tier
    • inspect the Actions tab
    • decide which workshop path fits
    • open Step 1 in a new tab
  • Preserve the original orientation intent while making each item produce an observable outcome.

• Add one more interactive snippet

  • Add a browser-ready URL snippet learners can use directly during the warm-up.
  • This raises interaction density without introducing local-tool requirements.

• Tighten checkpoint outcomes

  • Expand the checkpoint so it confirms not just understanding of the workshop goal, but also path choice, applicability of Step 1 guidance, and the next file to open.

Example of the updated warm-up pattern:

## Quick check: where are you starting from?

Complete this 3-minute warm-up. Open your [GitHub billing summary](https://github.com/settings/billing/summary). Then work through the checklist to determine your next steps.

- [ ] I noted the plan name shown on that billing page
- [ ] I picked the row in the table above that best matches how I want to work today
- [ ] I opened the **Actions** tab in any public repository
- [ ] I opened [Step 1](01-prerequisites.md) in a new tab so I know where I'll continue next