Define design-doc expectations for deep, high-risk PRs

Desired Behavior

The SDK repository's code-review guide should ask for a concise design doc when a deep, high-risk SDK or Agent Server change cannot be evaluated efficiently from the diff, while avoiding unnecessary documentation for small or self-explanatory changes.

The policy must also account for .pr/ cleanup: an automated approval must not remove the only design artifact before a human maintainer has had the opportunity to review it.

Acceptance Criteria

• Deep, high-risk PRs without adequate design context receive a COMMENT review instead of automated approval.
• Low-risk, trivial, generated, or self-explanatory changes are not blocked solely by design-doc absence or line count.
• An equivalent PR-description write-up has clear minimum content: intent, important before/after behavior or API shape, compatibility/risk, and grounded code references.
• A .pr/ design doc remains available through the human review decision, or equivalent persistent context is present in the PR description before automated approval triggers cleanup.
• The guidance remains consistent with the SDK repository's APPROVE-or-COMMENT policy and eval-risk rules.

This issue was created by an AI agent (OpenHands) on behalf of @enyst.

Start with the SDK repository's code-review guide and the .pr/ cleanup workflow, then trace the existing APPROVE-or-COMMENT and eval-risk guidance. Update the documented expectations so high-risk changes require sufficient design context while trivial changes are not blocked. Done means the acceptance criteria are covered and design context remains available through human review.