Say when a session is working, and what it was told

Three things a phone could not see, all of them the same shape: the
session was doing something and nothing on screen said so.

A turn nobody here started never reported itself. `Running` was sent
where a message was *sent*, so a session picked up mid-turn, one
compacting on its own, or one another agent wrote to sat there reading
as idle until it finished. The driver now says it from what it observes
-- output that could only come from a turn in flight -- which is the
same set of events that already announced a steer, with the ends
swapped.

An imported session had it worse: nothing but replayed lines ever
reaches it, and a status was not among them, so it was permanently
whatever it was when it was adopted. Its file does not record a turn
ending, but it does record why each assistant message stopped, and
`tool_use` versus anything else answers it. A record that says nothing
leaves the status alone rather than voting for idle.

Messages from other agents were dropped outright: the CLI marks them
meta, and this replayed everything except meta. They are now a row of
their own, closed by default like a tool call, named for the session
that sent it -- not the reader's own bubble, because they did not say
it, and a session working on something this phone never asked for is
exactly what one of these explains.

Measured against a real session file rather than guessed: the peer
record carries the sender's name and the message body in `origin`,
beside a copy wrapped for the model to read.

server/src/session/driver.rs

@@ -104,6 +104,19 @@ pub enum Event {
         #[serde(default, skip_serializing_if = "Option::is_none")]
         about: Option<String>,
     },
+    /// A message another agent sent this session.
+    ///
+    /// Its own kind rather than a `UserMessage`, because it is not
+    /// something the reader said and a transcript that renders it in their
+    /// voice is claiming they did. It also explains what would otherwise
+    /// be inexplicable: a session that starts working on something nobody
+    /// on this phone asked for.
+    PeerMessage {
+        /// The sending session's own name, which is what the reader
+        /// recognises it by -- the socket path it came from is not.
+        from: String,
+        text: String,
+    },
     /// The manager's record of a question being answered, so a rendered
     /// question card resolves on every device, not just the one that
     /// answered it.