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/stores/session-store.ts

@@ -94,6 +94,9 @@ export interface Agent {
   title: string | null;
   cwd: string;
   model: string | null;
+  requiresAttention?: boolean;
+  attentionReason?: "finished" | "error" | "permission" | null;
+  attentionTimestamp?: Date | null;
 }
 
 export interface Command {
@@ -816,6 +819,9 @@ export const useSessionStore = create<SessionStore>()(
           lastActivityAt,
           cwd: agent.cwd,
           provider: agent.provider,
+          requiresAttention: agent.requiresAttention ?? false,
+          attentionReason: agent.attentionReason ?? null,
+          attentionTimestamp: agent.attentionTimestamp ?? null,
         });
       }
       return entries;