Gate Step 6 install on verified gh authentication

[Copilot]

Step 6 had the highest dropout point because gh-aw install implicitly requires authenticated gh, but the flow only surfaced CLI presence and gave weak recovery guidance. This PR makes auth an explicit prerequisite and adds in-place recovery where failures occur.

• Prerequisites now include authenticated gh as a hard gate

  • Updated workshop/01-prerequisites.md pre-flight checklist to include gh auth status.
  • Added explicit gh auth login command and expected success signal.
  • Updated Step 1 checkpoint to require verified authentication, not just installation.

• Step 6 pre-flight now checks installability conditions, not just binary presence

  • Replaced the pre-flight block in workshop/06-install-gh-aw.md with explicit install + auth checks.
  • Added a prominent caution block that tells learners exactly how to recover if auth is missing.

• Failure recovery is now local to the failing step

  • Step 6 now contains direct “do this now” guidance (gh auth login) so users can self-recover without context switching.

gh --version && echo "✅ gh installed" || echo "❌ install gh first"
gh auth status && echo "✅ gh authenticated" || echo "❌ run: gh auth login"