tracking: Validate closing Issue hierarchy readiness

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Background

The OpenAI PR review workflow now requires the native closing-Issue set to cover every open sub-issue before a parent can be closed. This Task tracks the child-owned documentation and validation evidence for that hierarchy rule.

Goal

Track the complete, non-overlapping child work needed to document and validate closing-Issue hierarchy readiness.

Non-goals

• Owning the documentation change or its Code Changes Tree.
• Defining child-level implementation or validation steps.
• Changing branch-protection settings.

Sub-issues

• docs: Add closing hierarchy validation procedure #17 — owns the documentation change and two-stage live validation.

Completion Criteria

• Every native child Issue is complete and closed.
• Child-owned evidence demonstrates both the incomplete and complete Close Set outcomes.
• No child-owned implementation detail remains duplicated in this Task.