refactor(chat): show line-based thinking summary instead of full text (#2774)

Keep thinking panel collapsed during streaming by removing forceOpen.
Summary now shows last reasoning line while streaming (latest thought)
and first line when done (topic), instead of the full concatenated text.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tlgimenes

apps/mesh/src/web/components/chat/message/assistant.tsx

@@ -118,15 +118,23 @@ function ThoughtSummary({
       : "Thought"
     : "Thought";
 
+  // Join with newlines (not spaces) so we can extract individual lines
   const rawText = parts
     .map((p) => p.text ?? "")
-    .join(" ")
+    .join("\n")
     .trim();
+  const lines = rawText.split("\n").filter(Boolean);
+
+  // Streaming: show last line (latest thinking). Done: show first line (topic).
+  const summaryLine = isStreaming
+    ? (lines[lines.length - 1] ?? "")
+    : (lines[0] ?? "");
+
   const summary =
-    !allPartsRedacted && rawText
-      ? rawText.length > 100
-        ? rawText.slice(0, 100) + "…"
-        : rawText
+    !allPartsRedacted && summaryLine
+      ? summaryLine.length > 100
+        ? summaryLine.slice(0, 100) + "…"
+        : summaryLine
       : undefined;
 
   const fullText = parts.map((p) => p.text ?? "").join("\n\n");
@@ -150,7 +158,6 @@ function ThoughtSummary({
       state={isStreaming ? "loading" : "idle"}
       detailVariant="prose"
       latency={latency}
-      forceOpen={isStreaming}
     />
   );
 }