No way to restrict which agent types the task tool can dispatch per custom agent. #2812

Summary
There is no supported way to limit which agent types a given custom agent may delegate to through the task tool. The agent_type enum of the task tool is built per session. It lists every built-in agent and every discovered or registered custom agent, whichever agent is calling.

A frontmatter property like agents: [...] would be the natural place to configure this,same as vscode agent mode discovers it (https://code.visualstudio.com/docs/agent-customization/custom-agents). Now It is not supported in either of the two ways agents are defined:

1.Auto-discovered .github/agents/*.agent.md with enable_config_discovery=True. "agents" property in agents.md is silently ignored.

2.Manual custom_agents=[...]: CustomAgentConfig has no "agents" field, and an extra "agents" key is silently dropped.

Verified in Environment:
github-copilot-sdk 1.0.16 (Python), pinned CLI 1.0.90
Linux, Python 3.12

Expected behaviour
One of these should exist:

-With enableConfigDiscovery=True: agents: [worker-a] in an .agent.md limits the agent_type values the task tool accepts when that agent calls it. The tool schema and the runtime validation ("Unknown agent_type … Valid types are") would both reflect it.
-SDK config: CustomAgentConfig gains the same field (for example agents: list[str]), mapped to the CLI wire format.