[Index / structure] bridgelet-product-audit/postmortems/README.md: Postmortems subfolder index

[phertyameen]

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

Purpose:
Create an index file listing every postmortem with a one-line summary and theme tag.

This file should cover:

• Group entries by theme (documentation-accuracy gaps, repository hygiene, unverified/open questions, structural observations).
• Note that several entries here are explicitly framed as open verification tasks rather than confirmed findings — flag which is which in the index.
• Keep the index scannable for someone doing a broader retrospective across many findings at once.

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 (99/100).

[OAKVISUALZ]

Please assign me this issue.