Publish preview releases, so installs and updates can be tested before merge #437

Problem

J5 Release only publishes a stable version and always marks it as the latest release. So a change to installing or updating can't be tested end to end until it has merged and shipped to everyone. The j5 CLI stack (#414–#418) hit this: an in-app server update across the executable rename can't be exercised before 0.0.48 is released.

The code already understands a preview channel, inherited from upstream: versions <x.y.z>-preview.<yyyymmdd>.<n>, T3CODE_CHANNEL=preview in the installer, j5 update --channel preview, and desktop preview builds with no update feed. Only J5's release workflow is missing it.

Done when

• J5 Release accepts a preview version and publishes it as a GitHub pre-release that is never marked latest.
• The stable installer URL, j5 update, and desktop updates never pick a preview up.
• The packaging runbook says how to cut, install, and remove a preview.

Requested by Jackson, 2026-10-03.