feat(maestro): rename a session over PATCH /api/sessions/:id

The daemon trims and caps the name, persists it to the sidecar under the
sessions lock, and broadcasts SessionUpdated. A blank name clears
metadata.name so the card falls back to the command, matching the rule the
spawn form already uses.

Refs #125

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/maestro-daemon/src/session.rs

@@ -716,6 +716,38 @@ impl SessionManager {
             .map_err(SessionError::Tmux)
     }
 
+    /// Rename a session (#125): set `metadata.name`, or clear it when the
+    /// requested name is blank so the session falls back to its command.
+    ///
+    /// The new name is persisted to the sidecar before the event goes out — a
+    /// rename the operator can see but that a daemon restart would undo is
+    /// worse than one that failed outright.
+    pub async fn rename(&self, id: Uuid, name: &str) -> Result<Session, SessionError> {
+        // The sidecar write happens under the sessions lock so what is on disk
+        // and what is in memory can never disagree about the newest rename.
+        let session = {
+            let mut sessions = self.inner.sessions.lock().await;
+            let rec = sessions.get_mut(&id).ok_or(SessionError::NotFound(id))?;
+            let mut renamed = rec.session.clone();
+            match maestro_protocol::normalize_session_name(name) {
+                Some(name) => renamed
+                    .metadata
+                    .insert("name".into(), serde_json::Value::String(name)),
+                None => renamed.metadata.remove("name"),
+            };
+            self.inner
+                .sidecar
+                .write(&renamed)
+                .map_err(SessionError::Sidecar)?;
+            rec.session = renamed.clone();
+            renamed
+        };
+        let _ = self.inner.events.send(CatalogEvent::SessionUpdated {
+            session: session.clone(),
+        });
+        Ok(session)
+    }
+
     /// Accept an agent-reported activity state (#105), the only path that can
     /// produce `ActivityState::Error`. The report outranks the activity pass's
     /// own inference until its TTL lapses — so an agent that dies mid-run