[Index / structure] bridgelet-product-audit/checklists/README.md: Checklists subfolder index

[phertyameen]

Index / structure: bridgelet-product-audit/checklists/README.md

Purpose:
Create an index file listing every checklist with a one-line 'use this when' summary and link.

This file should cover:

• Group entries by whether they're one-time (onboarding/readiness) or recurring (periodic review) checklists.
• Note the literal-checkbox format convention so future checklist authors follow the same style.
• Keep the index itself free of any actual checklist items — it should only route readers to the right document.

Scope: documentation-only addition to a new, standalone folder. No changes to frontend/ or mobile/ are needed to close this issue.

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Part of the bridgelet-product-audit/ knowledge-base initiative (98/100).

[OAKVISUALZ]

Please assign me this issue.