[Manohar Paturi (ManoharPaturi)]

Motivation & Context

The description sidecar name stripped a file's extension before appending the suffix, so notes.md and notes shared one sidecar. Writing either file's description overwrote the other's, rewriting one without a description deleted the other's description, and deleting either file removed both. The mapping from memory files to their sidecars needs to be injective.

Description & Review Guide

• What are the major changes? The suffix now appends to the full file name, so notes.md gets notes.md_description.md while extension-less names keep their existing name_description.md sidecar. Reads fall back to the legacy stripped name so descriptions written by earlier versions keep feeding the index, and writes and deletes retire both spellings so a legacy sidecar can never resurrect stale text.
• What is the impact of these changes? Each memory file owns exactly one description sidecar, existing stores keep their extension-less sidecars unchanged, and extension-bearing files upgrade to the injective name on their next write while their legacy description stays visible until then.
• What do you want reviewers to focus on? The fallback being read-only until a write retires the legacy sidecar, and the internal-file detection still hiding both spellings from listings.

Related Issue

Fixes #9871

AI Assistance

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Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR links to an agreed issue with no competing open PR, or the Related Issue section documents a trivial-change or repository-automation exception.
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Manohar Paturi (ManoharPaturi)]

Sorry for skipping the review step. #9871 now uses the bug report template with the sidecar reproduction, and this stays closed until the team agrees.