Record why a version was published

As a person responsible for a live workflow, I want to record why I published a version, so that the history explains itself to whoever reads it next.

Part of #4852 (Sandbox DevX epic). Phase 2, builds on version recording (#4866).

Outcome

Going live, promoting and restoring each offer a place to say why, and what was written shows beside the version in the history.

A version currently carries its number, who published it, when, and which sandbox it came from. It carries no reason. "Fixed the date parsing after the upstream upgrade" has nowhere to live, so it lives in someone's memory or in a chat message, and is gone by the time anyone needs it. For an agency that has to account for changes to a production system, a release with no reason is half a record.

Scope

• An optional note on a version, written at the moment it is published.
• Shown in the version list and wherever a version is named.
• Optional, not required. Making it mandatory turns it into a field people type a full stop into.

Notes

Worth deciding whether an empty note is worth prompting for on a restore, where the reason is usually the interesting one.

Start by reading #4866 for the existing version-recording work, then review the publishing, going-live, promotion, and restore flows described here. Done means an optional note can be recorded at publication, appears beside the version in the version list and wherever the version is named, and its empty-note behavior on restore is decided.