custom agents: support for `infer` property

[aeschli]

The GithHub Custom agent spec https://github.com/microsoft/vscode-internalbacklog/wiki/sweagentd%E2%80%90spec.md
defines an attribute infer.

When set, the custom agent is advertised as a subagent

[aeschli]

Implemented by #278971

• enable chat.customAgentInSubagent.enabled
• have some custom agents, e.g.

---
name: github-me
description: This agent has access to the Github MCP server. Use it for searching issues in Github repositories and other Github-related tasks.
tools: ['read', 'search', 'web', 'github/*']
argument-hint: The task to complete
---

I work with the following Github repositories:

vscode: microsoft/vscode
vscode-docs: microsoft/vscode-docs

My id is aeschli.

Use the github/* tools to manipulate issues, pull requests, and other Github resources in these repositories.

• in regular agent mode, turn off all access to the github tools, enable the agent tool
• ask 'what subagents can you use?'
• ask 'Can you find me all issues assigned to me that are about tool sets and create me a nice summary?'
  -> This will (hopefully) use the 'gihub-me' subagent