[Feature]: Make automated reviewers read repository guidance

[smolpaws]

Problem or Motivation

Automated PR reviewers do not consistently inspect repository-owned guidance before reviewing. Root and nested AGENTS.md files, CONTRIBUTING.md, and other review or coding guidelines can therefore be missed.

Desired Behavior

Both the GitHub PR reviewer automation and the /codereview plugin explicitly require the reviewer to read and apply repository guidance before reviewing changed code.

Acceptance Criteria

• The automation prompt requires reading root and applicable nested AGENTS.md files.
• The automation prompt also points to other relevant contribution and review guidance.
• The /codereview plugin prompt carries the same requirement.
• Tests cover both prompt surfaces.
• Generated automation artifacts remain in sync.

Alternatives Considered

Rely only on always-on workspace context or one configured review-guide path. Those mechanisms do not guarantee discovery of nested guidance and other repository documentation across both reviewer entry points.

Additional Context

Tracks OpenHands/extensions PR #474.

[github-actions]

✅ ready-for-dev label applied

This issue meets the readiness criteria and is ready for contribution.
A contributor can now open a PR that links this issue (Fixes #<number>).

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