Take subagent reports out of the main transcript, and separate turns with a rule

A row per finished background task is a screenful of dividers about work the
reader was not asking after, and one of them turned out to be a whole shell
command drawn as centred prose, because its words came from somewhere with no
reason to keep them short. `Event::TaskNote` is gone entirely, along with the
row that drew it. A subagent's closing report is recorded as that subagent's
own transcript's closing text and is read in the subcard, which is where it
was already going; what the parent gets a row for is a message a subagent
genuinely sends it, which arrives by the peer path and has had one all along.

What remains is the actual defect and the smallest thing that fixes it. The
fold still refuses to grow a settled reply, so a turn boundary is always a
message boundary, and where two replies then abut it puts a `TurnBreak`
between them: a hairline, no words, no colour. Made by the fold rather than
sent by the server, because it is not something that happened -- it is the
boundary between two things that did. `joinPages` puts one in at a page seam,
which the fold never gets to see.

The task notification is still what closes a task in `Status::Waiting`'s
bookkeeping, and the registry lookup that recognises one this translator never
saw start is what makes that work for a session adopted across a restart.

Verified on the emulator: three replies, three rules, and nothing about the
helpers anywhere in the parent. 170 server tests, 85 JVM tests, ktfmt, clippy,
rustfmt and Android lint clean.

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

app/androidApp/src/main/kotlin/com/example/aiapp/Dividers.kt

@@ -26,18 +26,9 @@ import java.time.format.FormatStyle
  * two it was is said by the words and the colour.
  *
  * The rules take [color] too, so the whole divider reads as one mark of one kind.
- *
- * [trailing] is drawn inside the rules, beside the words -- the one divider that opens onto
- * something needs its chevron there, and giving it its own copy of this layout is how the two would
- * come to sit at different heights.
  */
 @Composable
-fun TranscriptDivider(
-    text: String,
-    color: Color,
-    modifier: Modifier = Modifier,
-    trailing: (@Composable () -> Unit)? = null,
-) {
+fun TranscriptDivider(text: String, color: Color, modifier: Modifier = Modifier) {
     Row(
         verticalAlignment = Alignment.CenterVertically,
         horizontalArrangement = Arrangement.spacedBy(8.dp),
@@ -45,11 +36,32 @@ fun TranscriptDivider(
     ) {
         HorizontalDivider(Modifier.weight(1f), color = color)
         Text(text, style = MaterialTheme.typography.bodySmall, color = color)
-        trailing?.invoke()
         HorizontalDivider(Modifier.weight(1f), color = color)
     }
 }
 
+/**
+ * The rule between two replies that met with nothing said in between -- see
+ * [TranscriptItem.TurnBreak].
+ *
+ * No words and no colour. Every other divider here reports something that happened and is worth
+ * finding by scanning; this one only says "these are two", and it appears once per turn that
+ * started without anybody typing. Saying more was a screenful of announcements about background
+ * work the reader was not asking after -- one of them a whole shell command, drawn as centred prose
+ * because the words came from somewhere that had no reason to keep them short.
+ *
+ * The outline colour is the scheme's one for structure rather than for meaning, which is what this
+ * is. Inset from both edges so it reads as a separator between two rows rather than as the top edge
+ * of the one under it.
+ */
+@Composable
+fun TurnBreakRow(modifier: Modifier = Modifier) {
+    HorizontalDivider(
+        modifier.fillMaxWidth().padding(horizontal = 48.dp, vertical = 6.dp),
+        color = MaterialTheme.colorScheme.outlineVariant,
+    )
+}
+
 /**
  * The mark a clear leaves.
  *