feat: add parentAgentId field to server types

Add parentAgentId field to enable agent parent/child hierarchy:
- ManagedAgentBase in agent-manager.ts
- AgentSessionConfig in agent-sdk-types.ts
- AgentSnapshotPayloadSchema in messages.ts
- toAgentPayload() in agent-projections.ts

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

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

packages/server/src/server/agent/agent-manager.ts

@@ -94,6 +94,7 @@ type ManagedAgentBase = {
   lastUsage?: AgentUsage;
   lastError?: string;
   attention: AttentionState;
+  parentAgentId?: string;
 };
 
 type ManagedAgentWithSession = ManagedAgentBase & {