fix: unify session persistence to preserve name on restart

- flushSave and SaveSession now use PersistSession instead of
  agent.SaveFull, ensuring the multi-agent format with Name field
  is consistently written
- Rename now calls PersistSession immediately so renamed sessions
  survive server restart

The previous approach had two save formats (legacy single-agent via
agent.SaveFull, and new multi-agent via PersistSession) writing to
the same file. Since flushSave ran on every turn, it would overwrite
the Name field with the legacy format that didn't preserve it.

session/registry.go

@@ -138,6 +138,9 @@ func Rename(oldName, newName string) error {
 	delete(sessions, oldName)
 	mu.Unlock()
 
+	// Persist immediately so the rename survives restart
+	PersistSession(newName)
+
 	pkgLog.Info("renamed session %s -> %s", oldName, newName)
 	PublishEvent("session."+sess.ID()+".rename", oldName+" "+newName)
 	return nil

session/session.go

@@ -361,26 +361,16 @@ func (a *Session) flushSave() {
 	if !dirty || a.id == "" || a.sessionsDir == "" {
 		return
 	}
-	path := a.activeSessionPath(a.id, ".json")
-	if err := os.MkdirAll(filepath.Dir(path), 0700); err != nil {
+	// Use PersistSession for consistent format (writes Name field correctly)
+	if err := PersistSession(a.Name()); err != nil {
 		a.log.Error("session save: %v", err)
-		return
-	}
-	if len(a.agents) > 0 {
-		ag := a.agents[0]
-		if err := ag.SaveFull(path, a.id, a.Name(), a.CWD(), a.remote); err != nil {
-			a.log.Error("session save: %v", err)
-		}
 	}
 }
 
-// SaveSession writes the current session state to the given path.
-// Returns nil if there is no agent (empty session).
+// SaveSession writes the current session state.
+// The path argument is ignored; session is saved via PersistSession.
 func (a *Session) SaveSession(path string) error {
-	if len(a.agents) > 0 {
-		return a.agents[0].SaveFull(path, a.id, a.Name(), a.CWD(), a.remote)
-	}
-	return nil
+	return PersistSession(a.Name())
 }
 
 // --- Identity accessors ---