Update agent lastActivityAt on stream events

Problem: lastActivityAt was only updated when agent_state messages arrived
(which only happens on agent lifecycle changes), not when the agent was
actively streaming output. This meant the agent directory showed stale
"last activity" times.

Solution: Update agent.lastActivityAt whenever agent_stream events arrive,
using the event's timestamp. Now lastActivityAt tracks real-time activity.

Updates happen on:
1. agent_state message → lastActivityAt = updatedAt (from server)
2. agent_stream message → lastActivityAt = event.timestamp (real-time!)

This ensures the agent directory always shows fresh activity times.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

packages/app/src/contexts/session-context.tsx

@@ -628,6 +628,17 @@ export function SessionProvider({ children, serverUrl, serverId }: SessionProvid
         next.set(agentId, false);
         return next;
       });
+
+      // Update agent's lastActivityAt to track stream activity
+      setAgents(serverId, (prev) => {
+        const agent = prev.get(agentId);
+        if (!agent) {
+          return prev;
+        }
+        const next = new Map(prev);
+        next.set(agentId, { ...agent, lastActivityAt: parsedTimestamp });
+        return next;
+      });
     });
 
     const unsubAgentStreamSnapshot = ws.on("agent_stream_snapshot", (message) => {