feat: auto-inject parentAgentId in MCP create_agent tool

When an agent calls the MCP create_agent tool to spawn a child agent,
the parent-child relationship is now automatically established:

- Added setManagedAgentId() to AgentSession interface (optional method)
- ClaudeAgentSession stores its managed agent ID and includes it as
  X-Caller-Agent-Id header when connecting to the agent-control MCP
- AgentManager.registerSession() calls setManagedAgentId() after
  registration so the session knows its ID before first prompt
- MCP server extracts X-Caller-Agent-Id header from init request and
  stores it as callerAgentId
- create_agent handler auto-injects callerAgentId as parentAgentId
  if not explicitly provided

This enables parent agents to spawn child agents that are correctly
associated, so they appear in the parent's Sub-Agents menu instead
of on the homepage.

🤖 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

@@ -706,6 +706,9 @@ export class AgentManager {
       throw new Error(`Agent with id ${agentId} already exists`);
     }
 
+    // Inform the session of its managed agent ID for MCP parent-child relationships
+    session.setManagedAgentId?.(agentId);
+
     const managed = {
       id: agentId,
       provider: config.provider,