I'm liking the idea of a per-issue key/value store.

I think keys and values are both strings, with possible further
parsing/interpretation for some keys?

For the application in this issue we'd teach ent to create a "finished"
key, maybe when setting the state to Done, with a value of an
ISO8601-style datetime stamp (e.g. "2025-07-12T15:08:58-06:00").

There would be a documented spec of reserved keys and their special
meanings and handling.

Full user access via CLI, maybe something like `ent get VARIABLE` and
`ent set VARIABLE VALUE`?  Or `ent variable [NAME[=VALUE]]`?

Or maybe merge this with the tags somehow?  Would that be simpler,
or more complicated (for the user)?