Clarify archetype metadata by removing difficulty references

[Copilot]

The issue was a documentation inconsistency in archetype front matter: level is the valid property, but some archetype comments still referred to difficulty. This change removes those stale references so the metadata examples are consistent across archetypes.

• Archetype metadata cleanup

  • Updated inline front matter comments in test-related archetypes to describe level correctly.
  • Standardized wording so archetype examples no longer imply that difficulty is a supported property.

• Files updated

  • archetypes/test.md
  • archetypes/optional-test.md
  • archetypes/final-test.md

• Example

  - level: "beginner" # Difficulty level of the test
  + level: "beginner" # Level of the test