Update core-concepts-variables-agents-channels lab (#133)

- Change duration from 55 to 30 minutes (5 + 13 + 12)
- Rewrite Variables UC to exploration-only (no creating/modifying)
- Add CAREful Prompts PDF download for child agent knowledge
- Add / reference technique for child agent in orchestration instructions
- Clarify orchestration steps are done in parent agent Overview page

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: Dewain27

lab-config.yml

@@ -114,7 +114,7 @@ labs:
   core-concepts-variables-agents-channels:
     title: "Master Variables, Multi-Agent Architectures, and Channel Deployment"
     difficulty: "Intermediate"
-    duration: 55
+    duration: 30
     section: core
     order: 175
     journeys: [quick-start, business-user, developer]