Make PR descriptions concise and review-directed #240

Outcome

A reviewer should find the result, design boundary, highest-risk review questions, and evidence without wading through repeated headings or metadata checklists.

Scope

• Replace the current PR template with four focused sections: Outcome, Design and boundaries, Review focus, Evidence and remaining gates.
• Keep the existing knowledge-sharing and GitHub metadata contract, but scale narrative depth to risk and manage metadata in GitHub.
• Document the change in docs/backlog/pr-review-quality.md and align the PR writing guide.

Acceptance

• Template still prompts for issue/backlog links, close/reference semantics, trust/failure/persistence where relevant, ordered code map, concrete review prompts, and residual gates.
• Documentation explicitly distinguishes required information from required headings.
• No product/runtime behavior changes.

Out of scope: rewriting historical PR descriptions or automating metadata checks.