feat: add "Requires Attention" agent grouping feature

Implement a new UI section that surfaces agents requiring user attention at the top of the agent list. This includes agents that are running, have pending permissions, or have errors.

**Backend changes:**
- Add `requiresAttention` field to agent projections and SDK types
- Implement logic in AgentManager to detect attention states (running, pending permissions, errors)
- Add WebSocket message handler to notify clients of attention state changes
- Update session state to include attention tracking

**Frontend changes:**
- Add "REQUIRES ATTENTION" section in agent-list component with visual prominence
- Update useAggregatedAgents hook to expose `requiresAttention` agents
- Enhance session store to track and update attention states
- Add real-time updates via WebSocket for attention state changes
- Update type definitions across agent directory and stream types

Verified working via Playwright MCP testing on localhost:8081.

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

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

packages/app/src/components/agent-input-area.tsx

@@ -125,6 +125,7 @@ export function AgentInputArea({ agentId, serverId }: AgentInputAreaProps) {
       on: () => () => {},
       sendPing: () => {},
       sendUserMessage: () => {},
+      clearAgentAttention: () => {},
       subscribeConnectionStatus: () => () => {},
       getConnectionState: () => ({ isConnected: false, isConnecting: false }),
     }),