CONTRIBUTING + adapter/task authoring guide

[macanderson]

Context. Adding an agent adapter or a task should be a ~20-minute job for an outside contributor.

Scope

• CONTRIBUTING.md: dev setup, the gates (typecheck / test / arena verify), PR expectations.
• "Add an adapter" walkthrough — the four hooks and the token-normalization rules.
• "Add a task" walkthrough — task.json + workspace/ + verify/ + solution/ and the discriminate invariant.

Acceptance. A newcomer adds a trivial adapter and task following only the docs.