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/hooks/useGateway.ts

@@ -94,6 +94,8 @@ export function useGateway() {
           outputTokens: s.outputTokens as number | undefined,
           channel: (s.lastChannel || s.channel) as string | undefined,
           kind: s.kind as string | undefined,
+          model: s.model as string | undefined,
+          agentId: s.agentId as string | undefined,
         })));
       }
     } catch {