feat: display model name badge in header for active session

Shows the model (e.g. claude-opus-4-6) as a subtle chip next to the
session label in the header. Hovering reveals agent ID if available.
Model and agentId are now extracted from sessions.list response.

src/types/index.ts

@@ -27,6 +27,8 @@ export interface Session {
   outputTokens?: number;
   channel?: string;
   kind?: string;
+  model?: string;
+  agentId?: string;
 }
 
 export type ConnectionStatus = 'disconnected' | 'connecting' | 'connected';