End a subagent on its own end_turn, not the parent's tool_result, and give the expander a touch-sized row

The Agent tool runs subagents in the background, so the parent's result
arrives at launch while the subagent works on for minutes; finishing on it
read a running agent as finished with a transcript cut off at launch. A
subagent now ends on its own message_delta end_turn, and a later line for a
finished one reopens it, since a background agent can be messaged again.

The card's expander row was only the chevron's height, so a tap for it
landed on the first subcard; it is the platform's 48dp minimum now.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

SUBAGENTS.md

@@ -50,17 +50,35 @@ There is no separate delete.
 2. Every child line is translated by that subagent's own `Translator`
    (one per subagent: tool ids are unique but streaming deltas are by
    content-block index, and parallel subagents interleave).
-3. When the parent's `tool_result` for the Task id arrives, the parent gets
-   its `ToolEnd` as before, and the subagent gets `Status Exited`.
-4. When the parent session's process exits (`Status Exited` on the
+3. **The parent's `tool_result` never finishes a subagent.** The Task tool
+   runs in the background by default: the `tool_result` -- "Async agent
+   launched..." -- arrives the moment it *starts*, while the subagent goes
+   on working for however long its own turn takes, sometimes minutes. What
+   ends it is its own turn ending: the raw API's `message_delta` on its
+   stream carrying `stop_reason: "end_turn"` (a `stop_reason` of `tool_use`
+   is the model about to call one, not an end), or a `result` line for its
+   own turn if a future CLI version ever sends one. Either maps to
+   `Status Exited`; the subagent's vocabulary has no `Idle`, so the
+   equivalent event `dispatch` produces for an ordinary session is dropped
+   rather than written. A shipped version of this finished on the
+   `tool_result` instead, which read a running background agent as
+   "finished" with its transcript truncated at the moment it launched.
+4. **A child line for a subagent that already finished reopens it**
+   (`Status Running`) rather than being dropped: a background Task can be
+   sent another message long after its first turn ended, and that is
+   exactly what a further line for it means. Same transcript, same child
+   `Translator`, just picking back up.
+5. When the parent session's process exits (`Status Exited` on the
    session), every subagent still `Running` gets `Status Exited` too: its
    process was the parent's.
 
 A subagent that was mid-flight when the backend restarted keeps working:
 the registry reopens the existing transcript on the next child line, and
-the file continues its sequence. If its Task call finished while the backend
-was down nothing ever closes it -- its last status stays `Running`, which
-the list reports as **unknown** rather than as running (see the wire shape).
+the file continues its sequence -- the same reopening #4 describes, whether
+what closed it was a restart or its own `end_turn`. If its turn ended while
+the backend was down nothing recorded that until the next line arrives, so
+its last status stays `Running`, which the list reports as **unknown**
+rather than as running (see the wire shape) until then.
 
 Title: the Task call's `description` input, then ` (<subagent_type>)` when
 one is given; falling back to the tool's name when the child arrives before
@@ -71,7 +89,7 @@ one is given; falling back to the tool's name when the child arrives before
 - `session/subagent.rs` -- the registry: `Subagents` (per session, in
   `Shared`), `Subagent` (its `Transcript` behind a mutex plus a
   `broadcast::Sender<SeqEvent>`), `record(id, event)`, `start(id, title,
-  prompt)`, `finish(id)`, `finish_all()`, `list()` from disk. Drivers get an
+  prompt)`, `finish(id)`, `reopen(id)`, `finish_all()`, `list()` from disk. Drivers get an
   `Arc<Subagents>` beside their `EventSink`; llama ignores it.
 - `session/claude/translate.rs` -- routes child lines by parent id, holds
   one child `Translator` per subagent, remembers pending Task calls'