Tell the model when a message was a steer

A message typed during a turn reaches the model at the next model call if
the turn has one left, and otherwise as the opening line of the next turn --
Claude's read out of the fifo after the turn ended, Codex's requeued when
turn/steer is refused. Read there it is indistinguishable from a reply, so
the model treats the answer it just gave as seen.

Both drivers now compose the text the CLI receives through
driver::message_body, which prefixes a note saying the message was written
without having seen the rest of that turn. The transcript still holds the
words that were typed; only the CLI's copy carries the note.

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

server/src/session/claude.rs

@@ -503,7 +503,7 @@ impl Driver for ClaudeDriver {
         // An image goes into the message itself; the model looks at it. Any
         // other file stays where the upload put it and the message says where,
         // because the CLI can read a file by path and a model cannot be handed
-        // a trace any other way. Named after the text, so the words come first.
+        // a trace any other way.
         let mut files = Vec::new();
         for id in &attachments {
             let sent = if crate::media::media_type_for(id).is_some() {
@@ -517,18 +517,6 @@ impl Driver for ClaudeDriver {
                 });
             }
         }
-        let mut body = text.clone();
-        for path in files {
-            if !body.is_empty() {
-                body.push_str("\n\n");
-            }
-            body.push_str(&format!("Attached file: {}", path.display()));
-        }
-        if !body.is_empty() {
-            content.push(json!({"type": "text", "text": body}));
-        }
-        let line =
-            json!({"type": "user", "message": {"role": "user", "content": content}}).to_string();
         let mut queue = self.queue.lock().unwrap();
         // Saying so beats writing into a fifo that nothing is reading, which is
         // what this used to do -- the message went nowhere and looked exactly
@@ -541,6 +529,15 @@ impl Driver for ClaudeDriver {
             });
             return;
         }
+        // Built under the lock because whether this is a steer decides what the
+        // CLI is told, and the answer must be the same one the branch below acts
+        // on -- see `super::driver::message_body`.
+        let body = super::driver::message_body(&text, &files, queue.running);
+        if !body.is_empty() {
+            content.push(json!({"type": "text", "text": body}));
+        }
+        let line =
+            json!({"type": "user", "message": {"role": "user", "content": content}}).to_string();
         if queue.running {
             // Into the running turn, now. Announced when the CLI shows it has
             // been round the model again -- see `Queue`.