docs: move YAML validation checkpoints earlier in Steps 7 and 11

[Copilot]

Steps 7 and 11 were exposing students to multi-section YAML with no in-editor feedback until the final validate step, causing 6.1% and 3.0% dropout rates respectively.

Step 7 (07-your-first-workflow.md)

• Add expected-output snippet (✔ hello-agent.md — valid) immediately after each incremental frontmatter sub-step (after on:, after permissions:) so students get a tight feedback loop at each authoring checkpoint
• Add VS Code editor setup tip (YAML extension, tabSize: 2, formatOnSave) in "Before You Start" to prevent tab-vs-space errors before they happen

Step 11 (11-build-daily-status.md)

• Promote "Common Mistakes" section to appear before "Build pass 1" — students now read the five common YAML pitfalls (tabs, missing ---, missing copilot-requests: write, etc.) before writing, not after failing
• Update the cross-reference from "section below" → "section above" to match the new position