docs: submit challenges as a reprex

[drmowinckels]

Summary

Reworks the community contribution model so challenges are submitted as a reprex (via the reprex package) rather than free-text task/example/solution fields.

Why: a reprex guarantees each challenge is well-stated in runnable R — no ambiguous, prose-only puzzles. It carries the premise (setup + what to write) and the expected outcome (printed output, or an image for visual challenges) in one reproducible block.

• Issue form — the reprex is the single required content field. Reference solution + tests are now optional (they help review/verification but aren't needed to submit). No working solution required in the reprex itself.
• CONTRIBUTING — a short Write your challenge as a reprex walk-through with a worked example, plus a note on visual challenges (attach the target image; keep the plotting code — the answer — to yourself).
• architecture — records that community challenges are reprex-shaped while AI drafts are structured fields, and both coexist in the queue (the post step renders whichever shape a challenge has).

For review

• Does this match what you had in mind for the submission format?
• Open question it surfaces for slice 3 (the post cron): it now needs to render two shapes — a community reprex block, or Jinx's structured prompt+sample. Worth deciding whether Jinx should eventually emit a reprex-style statement too, for one uniform shape.

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