feat: add parentAgentId to MCP create_agent tool

Add parentAgentId optional parameter to the create_agent MCP tool input
schema so agents can create child agents with explicit parent references.
Also update registerSession() to copy parentAgentId from config to the
managed agent object.

🤖 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

@@ -727,6 +727,7 @@ export class AgentManager {
       historyPrimed: false,
       lastUserMessageAt: null,
       attention: { requiresAttention: false },
+      parentAgentId: config.parentAgentId,
     } as ActiveManagedAgent;
 
     this.agents.set(agentId, managed);