merge a /clear chain into one conversation

The list collapses each chain to its newest link — the only one that can be
resumed — carrying the root's title and start time, the summed message count
and a part badge. The detail splices the chain's transcripts oldest-first with
a divider between parts, server-side, so the client's index-window pagination
needed no change.

The divider says "context cleared — nothing above this is in memory", because
the whole risk of merging is that the history reads as continuous when the
agent's context is not. Delete cascades the chain and the confirm says how many.

Supersedes the "continues X" line from the previous commit: there is nowhere to
link to once the parent is scrolled up above you.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/apps/officer-web/Screens/Dashboard/ChatHistory/index.tsx

@@ -111,7 +111,7 @@ export const SessionListPage = ({ isNew }: SessionListPageProps) => {
           initialOffset: detail.offset,
           cwd: detail.cwd,
           title: detail.title,
-          continuedFrom: detail.continuedFrom,
+          partCount: detail.partCount,
         });
       } catch (err) {
         if (cancelled) return;