Keep a subagent's words in its own transcript, and count the ones already running

Two corrections to the previous commit.

A subagent's closing report belongs in the subagent's transcript, which is
where it already is; drawing it as a card in the parent's put the same
paragraph in two places for a reader who did not ask for it. The row is a
divider now -- a boundary, which is what the transcript actually needed there
-- closed, saying only what reported and how it went. Opening it shows the
report anyway, since leaving the conversation to read one line has its own
cost, and a backgrounded command has no transcript of its own so this is the
only place its report exists at all: that one names itself from its summary
and has nothing left to open. `TranscriptDivider` grew a `trailing` slot for
the chevron rather than the row growing its own copy of the rules.

And the status was wrong for a session that was already running before the
update, which is every session when the backend is replaced under it.
Adoption picks a session's stdout back up from a recorded offset, so the
`task_started` lines for subagents launched earlier are behind it and the
translator never saw them -- it started with an empty set and reported `idle`
with a subagent plainly still working. `Subagents::any_open` reads the
directory instead, which is a measurement rather than bookkeeping and is right
for a session this process did not start. Both sources are kept and neither
subsumes the other: the translator's own set is the only thing that knows
about a backgrounded *command*, which has no subagent to be found. The same
pair decides whether an ending has already been reported, so a task that began
before the restart still gets its divider.

Echo's helpers now record their report as their own subagent's closing text,
the way the real driver does, so the fixture has the shape being tested.

Verified on the emulator: three dividers closed, one opened to its report, and
each reply drawn as its own message. 170 server tests, ktfmt, clippy, rustfmt,
Android lint and the JVM unit tests all clean.

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

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

@@ -26,9 +26,18 @@ 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) {
+fun TranscriptDivider(
+    text: String,
+    color: Color,
+    modifier: Modifier = Modifier,
+    trailing: (@Composable () -> Unit)? = null,
+) {
     Row(
         verticalAlignment = Alignment.CenterVertically,
         horizontalArrangement = Arrangement.spacedBy(8.dp),
@@ -36,6 +45,7 @@ fun TranscriptDivider(text: String, color: Color, modifier: Modifier = Modifier)
     ) {
         HorizontalDivider(Modifier.weight(1f), color = color)
         Text(text, style = MaterialTheme.typography.bodySmall, color = color)
+        trailing?.invoke()
         HorizontalDivider(Modifier.weight(1f), color = color)
     }
 }