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

Desired Behavior

The extensions repository's code-review guide should ask for concise design context when a deep, high-risk extension, skill, plugin, or integration change cannot be evaluated efficiently from the diff, while avoiding unnecessary documentation for small or self-explanatory changes.

Because this repository has the .pr/ artifact workflow but does not currently include the repository-local pr-design-doc authoring skill, the guidance should permit either an available design-doc tool or an equivalent PR-description write-up and must not promise unavailable local tooling.

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.
• Equivalent design context states intent, important before/after behavior or contract shape, compatibility/risk, and grounded code references.
• Guidance does not claim that .agents/skills/pr-design-doc/ exists in this repository.
• A .pr/ artifact is not the sole required context at the point automated approval triggers cleanup.

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

Locate the repository's code-review guide and the .pr artifact and automated-approval workflow, then read how context is evaluated and cleanup is triggered. Update the guidance so deep, high-risk changes require adequate design context, while low-risk or self-explanatory changes remain unblocked; verify it does not claim unavailable local tooling or make the artifact the sole required context.