docs: Add closing hierarchy validation procedure

[idy]

Background

• Parent: tracking: Validate closing Issue hierarchy readiness #16

The rollout guide needs a durable acceptance procedure for the native closing-Issue hierarchy gate so future workflow changes can be tested without rediscovering the expected sequence.

Goal

Document the two-stage live validation: a PR that closes a parent while omitting an open native child must fail, and the same PR must pass after the child is added to the native closing set.

Non-goals

• Changing the workflow implementation.
• Creating additional Check Run names.
• Testing non-native parent references or unrelated Issue links.

Code Changes Tree

README.md    # add the closing hierarchy live-validation procedure

Design

Add a concise validation case to the existing rollout guidance. The procedure must distinguish the initial missing-child failure from the final complete Close Set, name the expected OpenAI PR Review result for each stage, and state that already-closed children do not need to be repeated.

Keep the documentation descriptive: do not embed repository-specific test Issue numbers or transient run URLs.

Test And Acceptance Criteria

Acceptance Criteria

• The README describes the missing-open-child failure.
• The README describes the complete Close Set success.
• The README states that already-closed children are ignored.
• The change is limited to README.md.

Validation

• Run git diff --check.
• Observe OpenAI PR Review fail while the open child is absent from the PR's native closing set.
• Add the child to the closing set and observe all three OpenAI Checks pass.
• Confirm the final token table reports the unchanged parent Issue as reused.