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);

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

@@ -220,6 +220,12 @@ export async function createAgentMcpServer(
           .describe(
             "Run agent in background. If false (default), waits for completion or permission request. If true, returns immediately."
           ),
+        parentAgentId: z
+          .string()
+          .optional()
+          .describe(
+            "Optional parent agent ID. When set, this agent is a child of the specified parent agent."
+          ),
       },
       outputSchema: {
         agentId: z.string(),
@@ -246,6 +252,7 @@ export async function createAgentMcpServer(
       worktreeName,
       background = false,
       title,
+      parentAgentId,
     }) => {
       let resolvedCwd = expandPath(cwd);
 
@@ -265,6 +272,7 @@ export async function createAgentMcpServer(
         cwd: resolvedCwd,
         modeId: initialMode,
         title: normalizedTitle ?? undefined,
+        parentAgentId,
       });
 
       if (initialPrompt) {