Report a backgrounded command into the card that launched it

A backgrounded command has no subagent, so there is no second transcript for
its report to live in and its own tool card is the only record of it anywhere
-- and until the task notification arrives that card is showing the launch
result, which says the command is running. It was left saying that for ever.

The report now updates the call's own row (`Event::ToolUpdate` against its
tool_use id), so the card ends up holding what became of the command instead
of a claim nothing was ever going to correct. That includes the endings that
carry no summary: those are exactly the ones that went wrong, and a stale
"running in background" reads worst on them, so they say the status word
rather than nothing. A task with a subagent behind it is untouched and its
report stays where it was, in that subagent's own transcript.

Echo grew `/background [seconds]` for the shape end to end: the Bash call, the
launch result, a turn that ends `waiting`, and the completion arriving later
to correct the card and start a second turn.

Verified on the emulator: the card reads `Background command "sleep 5 && echo
done" completed (exit code 0)` where it had said "Command running in background
with ID: ...". 172 server tests, ktfmt, clippy, rustfmt, Android lint and the
JVM unit tests clean.

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

AGENTS.md

@@ -247,11 +247,16 @@ written, and the fold uses that same predicate to decide a reply is settled.
   screenful of dividers about work nobody was asking after, one of them a whole
   shell command. A subagent's report is its own transcript's closing text and
   is read in the subcard.
+- **A backgrounded command has no subagent, so its report lands in the tool
+  card that launched it** — a `ToolUpdate` against the call's own id, replacing
+  the launch result that says it is still running. `/background [seconds]` in
+  an echo session is that shape end to end.
 - **Two replies that meet are separated by a `TurnBreak`** — a hairline, no
   words. The reply that follows a turn boundary is a **new** message: the fold
   refuses to grow a settled reply, and without that the two ran together
-  mid-sentence. `./ui-sandbox.sh` plus `/subagent 3` in an echo session is the
-  whole rig; the helpers stagger a second apart so each reply is its own.
+  mid-sentence. `./ui-sandbox.sh` plus `/subagent 3` or `/background 5` in an
+  echo session is the whole rig; the helpers stagger a second apart so each
+  reply is its own.
 - **Whether work is outstanding has two sources and needs both.** The
   translator's `open_tasks` is what it watched start — the only thing that
   knows about a backgrounded command — and `Subagents::any_open` reads the