## Context

Renamed ThreadMetadataStore to SidebarThreadMetadataStore.
There was some confusion yesterday on what metadata should be stored in
`ThreadMetadataStore.
Only threads that should show up in the sidebar should be stored here
(effectively all non-archived ones). Hopefully the rename + doc comment
added makes this clearer.
Unfortunately we cannot move it into the sidebar crate since we need it
in the archived history view too.

## How to Review

This is purely a cosmetical change

## Self-Review Checklist

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- N/A