feat: add /new slash command to create chat sessions

Introduces a `/new` slash command that lets users create a fresh chat
session for the current agent without leaving the chat input.

- Add `createNewSession` to `useGateway` that calls `sessions.create`
  on the gateway (with a client-side fallback key when the RPC is
  unavailable).
- Register `/new` in the slash-command menu with i18n descriptions
  across all 8 supported languages.
- Wire `onNewSession` through `Chat` → `ChatInput` so typing `/new`
  triggers session creation.
- Add `extractAgentIdFromKey` and `formatAgentId` helpers to
  `sessionName.ts` to derive a human-readable agent name from the
  session key (e.g. `agent:my-cool-bot:…` → "My Cool Bot").
- Use the new helpers in `Header` and `App` to show per-session agent
  names, especially for sub-agent sessions where the gateway-level
  identity differs from the session agent.

Made-with: Cursor

src/lib/i18n.ts

@@ -189,6 +189,7 @@ const en = {
   'slash.model': 'Switch model for this session',
   'slash.compact': 'Compact conversation context',
   'slash.reset': 'Reset the session',
+  'slash.new': 'Create a new chat session',
   'slash.help': 'Show available commands',
 } as const;
 
@@ -358,6 +359,7 @@ const fr: Record<keyof typeof en, string> = {
   'slash.model': 'Changer de modèle pour cette session',
   'slash.compact': 'Compacter le contexte',
   'slash.reset': 'Réinitialiser la session',
+  'slash.new': 'Créer une nouvelle session de chat',
   'slash.help': 'Afficher les commandes disponibles',
 };
 
@@ -527,6 +529,7 @@ const es: Record<keyof typeof en, string> = {
   'slash.model': 'Cambiar modelo para esta sesión',
   'slash.compact': 'Compactar contexto de conversación',
   'slash.reset': 'Reiniciar la sesión',
+  'slash.new': 'Crear una nueva sesión de chat',
   'slash.help': 'Mostrar comandos disponibles',
 };
 
@@ -698,6 +701,7 @@ const de: Record<keyof typeof en, string> = {
   'slash.model': 'Modell für diese Sitzung wechseln',
   'slash.compact': 'Gesprächskontext kompaktieren',
   'slash.reset': 'Sitzung zurücksetzen',
+  'slash.new': 'Neue Chat-Sitzung erstellen',
   'slash.help': 'Verfügbare Befehle anzeigen',
 };
 
@@ -867,6 +871,7 @@ const ja: Record<keyof typeof en, string> = {
   'slash.model': 'このセッションのモデルを変更',
   'slash.compact': '会話コンテキストをコンパクト化',
   'slash.reset': 'セッションをリセット',
+  'slash.new': '新しいチャットセッションを作成',
   'slash.help': '利用可能なコマンドを表示',
 };
 
@@ -1036,6 +1041,7 @@ const pt: Record<keyof typeof en, string> = {
   'slash.model': 'Alterar modelo desta sessão',
   'slash.compact': 'Compactar contexto da conversa',
   'slash.reset': 'Redefinir sessão',
+  'slash.new': 'Criar uma nova sessão de chat',
   'slash.help': 'Mostrar comandos disponíveis',
 };
 
@@ -1205,6 +1211,7 @@ const zh: Record<keyof typeof en, string> = {
   'slash.model': '切换当前会话模型',
   'slash.compact': '压缩会话上下文',
   'slash.reset': '重置会话',
+  'slash.new': '创建新的聊天会话',
   'slash.help': '显示可用命令',
 };
 
@@ -1374,6 +1381,7 @@ const it: Record<keyof typeof en, string> = {
   'slash.model': 'Cambia modello per questa sessione',
   'slash.compact': 'Compatta il contesto della conversazione',
   'slash.reset': 'Reimposta sessione',
+  'slash.new': 'Crea una nuova sessione chat',
   'slash.help': 'Mostra comandi disponibili',
 };