Make the built-in research agent's MCP tools configurable

### Describe the feature or problem you'd like to solve

Research subagents sometimes need access to mcp servers to be productive

### Proposed solution

Today `definitions/research.agent.yaml` hardcodes tools: to github/* + web/grep/glob/view, so /research can't use my other configured MCP servers (ADO, etc.). Forking the agent /.copilot/agents or .github/agents works but freezes the prompt and misses upstream updates and can't reproduce research command's orchestration/report-export. Please add a setting (e.g.  research.additionalTools  /  research.mcpServers ) to extend the built-in research agent's allowed tools with selected MCP servers, so I keep research behavior + updates while adding my own data sources.

### Example prompts or workflows

_No response_

### Additional context

_No response_

從 definitions/research.agent.yaml 開始，追蹤內建研究代理的硬編碼工具如何載入，以及使用者設定如何處理。新增一個受支援的設定，以選取的 MCP 伺服器擴充允許的工具，同時保留內建研究行為、編排和報告匯出。

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