docs: add PR plan and requirements artifacts

[turisanapo]

Adds the PR plan and the requirements/implementation-plan documents for PR 4, 5A, and 7 to the repository root.

[coderabbitai]

Important

Draft PR not reviewed

Draft PRs are not automatically reviewed by default.

• Trigger a manual review

To automatically review draft PRs, update your CodeRabbit configuration:

reviews:
  auto_review:
    drafts: true

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