Replace <@USERID> tokens with @handles using Slack handle (name) as
preferred identifier, falling back to display_name then real_name.
Redesign thread rendering with bullet glyphs (● top-level, ◆ thread
parent), ├/└ reply branches, horizontal rules between roots, and an
ellipsis row for skipped replies. Resolve orphan thread parents from
the message cache when possible; otherwise render an explicit
"orphaned thread — parent unknown" box.
