Simplify Submission: drop type, namespace format, flatten content

[m-messer]

Summary

• Removes redundant type (TEXT/CODE/MATH/MODEL/OTHER) from Submission, FeedbackTarget, and ArtefactProfile; format alone now determines category via a namespaced, pattern-constrained convention (<category>:<subtype>, e.g. text:plain, code:sympy, math:sympy) — this also resolves the prior ambiguity where sympy/wolfram existed under both CODE and MATH.
• Flattens content's type-mirroring wrapper key ({ text: ... }, { expression: ... }, { code: ... }) to a single generic value key for every artefact type, with format-specific auxiliary fields (entryPoint, imports, notation, diagramType) as siblings.
• Deletes the now-unused ArtefactType.yml enum.
• Updates all evaluate.yml/chat.yml/evaluate-health.yml examples to match.

Closes #30. As a side effect, also resolves #28 (no documented fallback key for non-conventional content — value is now the universal key).

Test plan

• npm run lint passes (matches pre-existing warning baseline on main)
• npm run bundle succeeds

🤖 Generated with Claude Code

[maximiliansoelch]

Thanks for simplifying this. Code changes look good 👍