Chat ACL audit diffs rendered as `[object Object]` because the diff
viewer called `.toString()` on object values. Common chat operations
(archive, share) showed generic "updated chat" descriptions instead of
semantic ones.
Add `chatAuditLogDescription` to derive semantic descriptions from the
audit diff for successful chat writes: "archived/unarchived chat" for
archive toggles, "updated sharing for chat" for ACL-only changes.
Extract diff value formatting into `formatAuditDiffValue`, which renders
object values as deterministic compact JSON with sorted keys, fixing the
`[object Object]` rendering for chat ACLs and any other object-valued
fields. The previous `determineIdPSyncMappingDiff` workaround for IdP
sync mappings was removed because the generic formatting handles it.
Closes CODAGT-513
> Generated by Coder Agents on behalf of @johnstcn
Cian Johnston