sandboxes: add more metadata to version stamps

When we detect a divergence between two versions, we can tell users that there's been a change - but we can't say what it is.

If we can track the divergence down to a single version, or set of versions (and we should right? It's any version on main, not on staging, that comes after the last common version - it's the uncommon head), it would be nice to tell users "here's where the change occured"

We do have a bit of metadata already, and there's more we can trivially add

• The datetime the version was generated. This is basically the commit date.
• The source of the change, CLI or app. Maybe we can add github sync to this
• If a version was instigated by a user, maybe we can list what user

It seems Plausible that we can say: "the versions diverged on 10th January, triggered by Hunter in the app". That probably is enough to help a human know what to do

Start by tracing the sandbox version-stamp and divergence handling entry points; the issue does not name specific files or tests. Determine how the uncommon head is identified and where existing version metadata is stored, then establish which generated time, change source, and initiating user details should be persisted and shown.