Shift left learning-path packaging checks (framing, depends, website.yaml description + trailing newline)

[bonnywelsford-source]

Summary

Reviewers (see #418) keep re-finding the same Learning Hub packaging issues on LP PRs after Pathfinder CI is already green:

1. Framing / concept-intro packages listed in path milestones (should stay on disk for the website only)
2. First hands-on milestone still depends on framing
3. Missing path/step website.yaml description (required for Learning Hub meta / listings)
4. Missing trailing newline on website.yaml (deploy-preview concatenates type: docs after cat, which breaks Hugo YAML — same class of failure as #453 / #419)

This PR encodes those rules in a script + CI so authors catch them before review, and documents the single source of truth for authoring and the LP review skill.

What changed

Area	Change
.github/scripts/validate_learning_path_packages.py	New validator (framing, depends, description, trailing newline; no PyYAML dep)
.github/workflows/validate-json.yml	Wire validator into Validate Interactive Packages
docs/website-yaml-reference.md	Document conventions + local CI command
.cursor/skills/review-learning-path/reference-checks.md	Point Phase 2 at the script
.cursor/commands/create-learning-path/README.md	Authoring checklist alignment

Test plan

• python3 .github/scripts/validate_learning_path_packages.py passes on this branch
• Stripping the final newline from any website.yaml fails the script with a clear message
• Validate Interactive Packages job runs the new step on the PR

[bonnywelsford-source]

CI failure on https://github.com/grafana/interactive-tutorials/actions/runs/29296638582 was not from the new packaging script — it died earlier at Install dependencies for grafana-pathfinder-app.

Root cause: pathfinder-app now declares engines.node: >=24, and on Node 22 npm ci fails with Missing: typescript@5.9.3 from lock file. Bumped workflow Node to 24 in validate + deploy.