docs(codereview): require design context for deep PRs

[enyst]

• A human has tested these changes.

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Why

The extensions repository has the .pr/ artifact workflow but no repository-specific guidance for when deep, high-risk changes need design context. This adds the same review expectation being proposed in the other main repositories while accounting for extensions-specific contracts and the absence of a repository-local pr-design-doc skill.

Summary

• Define repository-owned signals for deep changes: skills, plugins, automations, integrations, manifest schemas, shared loading/validation, and cross-cutting migrations.
• Scale missing-design-context handling by risk without treating line count as an automatic gate.
• Define the minimum content for an equivalent durable PR-description write-up.
• Prevent automated approval from deleting the only .pr/ design artifact before a human maintainer reviews it.

Issue Number

Fixes #549

How to Test

Ran from the repository root:

git diff --check
grep -q '^triggers:' .agents/skills/custom-codereview-guide.md
grep -q 'temporary `.pr/` page' .agents/skills/custom-codereview-guide.md

All commands passed. This is a repository-local Markdown policy change with no runtime code path.

Video/Screenshots

Not applicable; there is no product UI change.

Notes

This deliberately does not claim .agents/skills/pr-design-doc/ exists in extensions. It allows any available design-doc artifact or an equivalent durable write-up and explicitly handles approval-triggered .pr/ cleanup.

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

@enyst can click here to continue refining the PR