[workshop-sim] Add prerequisite pre-flight guardrails to Steps 1 and 6

[Copilot]

Steps 1 and 6 were losing learners for the same reason: gh wasn’t reliably installed before gh-aw install time. This PR makes prerequisite checks explicitly blocking in Step 1 and adds a second early guardrail in Step 6.

• Step 1: make prerequisites operational, not informational

  • Added a copy-paste pre-flight block that checks gh, git, and optional node in one run.
  • Reframed prerequisites as explicit Required vs Optional with visual markers (✅ / ℹ️).
  • Kept platform install commands as direct one-liners for macOS, Ubuntu/Debian, and Windows.

• Step 6: add an early failure gate before install

  • Added an IMPORTANT pre-flight callout at the top of the step (same multi-check script pattern).
  • Added a one-line gh --version guard before extension install guidance.
  • Replaced generic “go back to Step 1” guidance with direct links to exact platform-specific install commands.

• Consistency/clarity refinements

  • Aligned wording so fallback/error text points to the correct install-command locations.
  • Kept shell snippets copy-paste safe for interactive terminals.

# Pre-flight check -- run this before proceeding
gh --version && echo "✅ gh CLI OK" || echo "❌ gh CLI missing — see install commands below"
git --version && echo "✅ git OK" || echo "❌ git missing"
node --version 2>/dev/null && echo "✅ node OK" || echo "ℹ️  node not required but useful"
echo "Pre-flight complete."