Allow /research mode to access configured MCP servers

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

I can configure MCP servers in Copilot CLI and use them from normal prompts, but when I run a `/research` prompt it appears that the research agent cannot see or access the same configured MCP server list/tool set. This is surprising because the MCP servers are configured for the session, and it makes research mode less useful for internal docs, catalog, or data-source MCP servers that are exactly the kind of sources I want research to use.

### Proposed solution

Please make `/research` inherit the configured MCP servers available to the main Copilot CLI session, or provide an explicit opt-in/allowlist for MCP servers that can be used by the research agent.

If this isn't possible, it would be helpful to make this limitation more visible/discoverable as it took me quite a while to figure out why my research sessions were failing to use the mcp servers.

### Example prompts or workflows

- `/research Use my configured internal docs MCP server to find the authoritative runbook for and summarize it with citations.`
- `/research Search my configured Datapedia/EngHub/internal catalog MCP servers for information about .`
- Let users mark specific MCP servers as research-safe/read-only so research reports can use them without needing custom workarounds.

### Additional context

This came up while troubleshooting MCP behavior in `/research`. Some tools appear to work there, but it does not look like the research agent has access to the same MCP server list that the main Copilot CLI session can see.

The key request is consistency and transparency: either make configured MCP servers available in research mode, or clearly document and surface the limitations so users know what to expect.