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_

Start with definitions/research.agent.yaml and trace how the built-in research agent's hardcoded tools are loaded and how user configuration is handled. Add a supported setting that extends the allowed tools with selected MCP servers while preserving the built-in research behavior, orchestration, and report export.

Written by the indexing model from the issue text.