Show a message another agent sent, on a session this server is running

Peer messages were only ever produced by the *import* path, reading them out
of the CLI's own session file. A message another agent sent to a session
this server was driving appeared nowhere, so the session started working on
something nobody on the phone had asked for and there was nothing on screen
to explain it.

Measured rather than guessed, because the obvious place to look for it is
empty: a real cross-session message sent to a real `--input-format
stream-json` session on CLI 2.1.237 produces **no `user` record**, and
nothing in the partial-message stream mentions it either. The whole of it
arrives as an `origin` object on the turn's `result`, in exactly the shape
the session file records -- so `import::peer_message` now reads both, one
function for one wire format. Two copies would drift the first time a field
is renamed, and the half that drifted would go on producing nothing, which
is indistinguishable from nobody having sent anything.

The cost is the position: the note lands after the reply it caused rather
than above it, because at no earlier point in the turn does the CLI say why
the turn started. Taken deliberately over the alternative -- a second reader
tailing the CLI's own session file for the one record stdout does not carry,
which is two sources of truth for one conversation and a poll per live
session. Recorded in PLAN.md so that if the CLI ever announces the injection
where it happens, the next reader knows to move it there.

Both halves tested: the real record shape, and an ordinary result carrying
no `origin` -- which is the half that decides whether the check is a check.
Four ordinary results on a real session's stdout had none between them.

AGENTS.md

@@ -273,6 +273,13 @@ first if a remote spawn ever mangles an argument.
   reply into the tool output under it -- and a container per row leaves
   whatever was drawn without one silently unselectable, which nothing on
   screen reports. Rows keep their tap handlers; selection is a long press.
+- **A message from another agent reaches a live session on the turn's
+  `result`, not before.** Measured on CLI 2.1.237 by sending a real
+  cross-session message to a real stream-json session: no `user` record, and
+  nothing in the partial-message stream -- the whole of it is an `origin`
+  object on the `result`, the same shape the session file records, which is
+  why `import::peer_message` reads both. So the note is drawn *after* the
+  reply it caused; that is the wire, not a bug. See PLAN.md.
 - **A queued message can be tapped to take it back**, which is
   `POST /sessions/{id}/unqueue` and a `messageDropped` event -- see PLAN.md's
   "Taking a queued message back". On a **Claude** session it always refuses,