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>

server/src/session/driver.rs

@@ -211,11 +211,15 @@ pub enum Event {
     /// A task the session started in the background reporting back: a
     /// subagent that has finished, or a backgrounded command.
     ///
-    /// Recorded because it is a message the session *received*, and without
-    /// it the turn it wakes up and runs has nothing in front of it. Two
-    /// replies then met with no row between them and were folded into one,
-    /// so a phone drew the answer to a question nobody could see as a
-    /// continuation of the previous sentence.
+    /// Recorded because the turn the session wakes up and runs would
+    /// otherwise have nothing in front of it: two replies met with no row
+    /// between them and were folded into one, so a phone drew the answer to
+    /// a question nobody could see as a continuation of the previous
+    /// sentence. It is drawn as a **divider** rather than as a message --
+    /// what it marks is the boundary, and the subagent's own words are in
+    /// the subagent's own transcript, which is where somebody who wants them
+    /// looks. Repeating them here would be the same text in two places, and
+    /// the copy is the one that goes stale.
     ///
     /// Its own kind rather than an update to the Task call's row: that row
     /// is wherever the call was made, which is above everything the session
@@ -236,6 +240,11 @@ pub enum Event {
         /// "finished" is the wrong word for a task that was killed.
         status: String,
         /// What it said on the way out, where it said anything.
+        ///
+        /// Only ever *shown* for a task with no [`TaskNote::title`], which is
+        /// a backgrounded command: it has no transcript of its own, so this
+        /// is the only record there is of it. A subagent's report is recorded
+        /// as that subagent's own closing text and is not repeated here.
         #[serde(default, skip_serializing_if = "Option::is_none")]
         summary: Option<String>,
     },