docs: add documentation style guidance

[rlippmann]

What changed

• Added a Documentation style section to AGENTS.md.
• Added a matching Documentation Style section to CONTRIBUTING.md.
• Guidance is scoped to docs wording only (behavior-first explanations, plain concrete terms, and explicit boundaries for spec/contract docs and fixture/eval-sensitive text).

Why

• Keep future documentation updates consistent with the newer behavior-first style.
• Make contributor expectations explicit so readability improvements do not weaken formal guarantees.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)