Add fast-track skip path for advanced users at Steps 4–5

[Copilot]

Advanced users familiar with GitHub Actions were dropping at Steps 4–5 with no mechanism to skip introductory material they already know.

Changes

04-github-actions-intro.md

• Replaces the old TIP (linked to Step 5) with a fast-track callout linking directly to Step 6
• Adds a Prerequisites Knowledge Check — 3-item checklist immediately after the TIP so users can self-assess before skipping

05-agentic-workflows-intro.md

• Moves the TIP callout to be the first actionable element after the heading (was previously below an italic intro line)
• Updates callout text to match the Step 6 fast-track pattern

Both files now open with:

> [!TIP]
> **Already know GitHub Actions?** Skip to [Step 6 — Install gh-aw](06-install-gh-aw.md).
> Come back here if you need a refresher on how agentic workflows differ from classic Actions.