[Index / structure] bridgelet-product-audit/runbooks/README.md: Runbooks subfolder index

[phertyameen]

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

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

This file should cover:

• Group entries by whether they're user-support triage, engineering diagnostics, or repository-maintenance procedures.
• Note that runbooks describe current behavior and operational workarounds, not proposed code changes.
• Keep the index scannable for someone handling a live support escalation who needs to find the right runbook fast.

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 (97/100).

[OAKVISUALZ]

Please assign me this issue.