codec: let open Json.Codec in cleanup across 10 more files
slack/{message,file,reminder,team,bin/login}, oci/spec/{index,intoto,
platform}, paseto, agent/todo. 8-14 usages each. slack/team and
slack/reminder records have `id` (and team also `name`); hoisted
`let id (t : t) = t.id` / `name` accessors and used `~enc:id` in
codecs. oci/spec/intoto subject record has `name`; uses `(s :
subject)` annotations on the enc lambdas. The rest are clash-free.