feat(maestro): name a session from its first prompt

POST /api/sessions/:id/prompt claims a session's one naming attempt and runs
claude -p --model haiku in the background; the answer is reduced to a short
slug and replaces the generated handle, settling the session at Haiku. A
chosen name, a rename mid-run, a missing binary or a timeout all leave the
handle alone. The prompt goes to the child on stdin and is never logged.

Refs #131

crates/maestro-daemon/src/session.rs

@@ -71,6 +71,11 @@ pub struct SessionRecord {
     /// (#115), so this is the observable that says its outcomes are safe to
     /// assert on. Daemon-local bookkeeping; never rides the wire.
     pub settled: bool,
+    /// Flipped when this session's one autonaming attempt is claimed (#131),
+    /// so two prompts in quick succession cannot both start a naming run.
+    /// Daemon-local bookkeeping; never rides the wire.
+    #[cfg(feature = "haiku-autoname")]
+    pub naming: bool,
 }
 
 #[derive(Debug, thiserror::Error)]
@@ -458,6 +463,8 @@ impl SessionManager {
                     bridge,
                     reported_activity: None,
                     settled: false,
+                    #[cfg(feature = "haiku-autoname")]
+                    naming: false,
                 },
             );
         }
@@ -661,6 +668,8 @@ impl SessionManager {
                     bridge,
                     reported_activity: None,
                     settled: false,
+                    #[cfg(feature = "haiku-autoname")]
+                    naming: false,
                 },
             );
             let _ = self
@@ -814,6 +823,63 @@ impl SessionManager {
     }
 }
 
+#[cfg(feature = "haiku-autoname")]
+impl SessionManager {
+    /// Claim this session's one autonaming attempt (#131).
+    ///
+    /// `Ok(false)` when there is nothing to name: the session already carries
+    /// a name a person chose or a previous prompt produced, or a naming run
+    /// is already in flight. Claiming and checking happen under the same lock
+    /// so two prompts arriving together cannot both win.
+    pub async fn claim_autoname(&self, id: Uuid) -> Result<bool, SessionError> {
+        let mut sessions = self.inner.sessions.lock().await;
+        let rec = sessions.get_mut(&id).ok_or(SessionError::NotFound(id))?;
+        let provisional = maestro_protocol::display::is_provisional_name(&rec.session);
+        if rec.naming || !provisional {
+            return Ok(false);
+        }
+        rec.naming = true;
+        Ok(true)
+    }
+
+    /// Replace a provisional handle with a name derived from the first prompt
+    /// and settle the session at [`NameSource::Haiku`] (#131).
+    ///
+    /// A rename that landed while the naming run was out wins: the check is
+    /// repeated here, under the lock, and a name a person chose is left alone.
+    pub async fn apply_autoname(&self, id: Uuid, name: &str) -> Result<(), SessionError> {
+        let Some(name) = maestro_protocol::normalize_session_name(name) else {
+            return Ok(());
+        };
+        let updated = {
+            let mut sessions = self.inner.sessions.lock().await;
+            let rec = sessions.get_mut(&id).ok_or(SessionError::NotFound(id))?;
+            if !maestro_protocol::display::is_provisional_name(&rec.session) {
+                return Ok(());
+            }
+            let mut named = rec.session.clone();
+            named
+                .metadata
+                .insert(NAME_KEY.into(), serde_json::Value::String(name));
+            named.metadata.insert(
+                NAME_SOURCE_KEY.into(),
+                serde_json::Value::String(NameSource::Haiku.as_str().into()),
+            );
+            self.inner
+                .sidecar
+                .write(&named)
+                .map_err(SessionError::Sidecar)?;
+            rec.session = named.clone();
+            named
+        };
+        let _ = self
+            .inner
+            .events
+            .send(CatalogEvent::SessionUpdated { session: updated });
+        Ok(())
+    }
+}
+
 /// Run a blocking `tmux` subprocess call on the blocking thread pool so it
 /// never stalls a tokio worker. `tmux` is a fork+exec that can take tens of
 /// milliseconds on a loaded host, and `send_keys` rides the per-keystroke hot