feat: add parentAgentId to client types and session store

Add parentAgentId field to:
- Agent interface in session-store.ts
- AgentDirectoryEntry in agent-directory.ts
- getAgentDirectory() includes parentAgentId
- normalizeAgentSnapshot() maps from server payload

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -139,6 +139,7 @@ function normalizeAgentSnapshot(snapshot: AgentSnapshotPayload, serverId: string
     requiresAttention: snapshot.requiresAttention ?? false,
     attentionReason: snapshot.attentionReason ?? null,
     attentionTimestamp,
+    parentAgentId: snapshot.parentAgentId,
   };
 }