Based on a partial revert of: 7bcdb12b4c5e9de0e4632c770336799e7c8d13d6

The main difference is instead of a feature flag we now check
`ReleaseChannel::Stable` != current_release_channel when the UI checks
if agent v2 (sidebar) features should be enabled.

Self-Review Checklist:

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

Closes #ISSUE

Release Notes:

- N/A or Added/Fixed/Improved ...

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Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>