Fix unsupported inline route template examples

[miguelcobain]

Updates Ember route examples to stop using inline <template> blocks inside Route classes imported from @ember/routing/route, which is not supported.

• Rewrites affected route examples to use:
  • app/routes/*.js for route logic
  • app/templates/*.gjs for templates
• Updates the route template guidance to recommend separate route and template files
• Extends the build/parser tooling so a single example can preserve multiple fenced code blocks
• Regenerates derived artifacts:
  • skills/ember-best-practices/AGENTS.md
  • packages/ember-best-practices-build/test-cases.json
  • skills/ember-best-practices.zip