[FEATURE]: Add @ mention syntax for one-time MCP server activation in chat #9548

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Add the ability to temporarily enable MCP servers for a single request using @ mention syntax in the chat interface.

Current behavior:

Currently, MCP servers are either globally enabled or disabled in settings. To use an MCP server, users must:

1. Navigate to settings
2. Enable the desired MCP server
3. Send their request
4. Optionally go back to settings to disable it

This creates friction when users only need an MCP server for a specific one-off request.
Why it would be beneficial:

Convenience: Quickly invoke MCP tools without changing global settings
Efficiency: Reduces context switching between chat and settings
Clarity: Makes it explicit when MCP tools are being used in a conversation

Suggested implementation:

Add @server_name syntax in chat to temporarily enable an MCP server for that request

Example: @github search for issues related to authentication
Example: @filesystem read the contents of config.json

The MCP server would only be active for that specific request
Show visual indication (e.g., badge or icon) that an MCP server was invoked
Support multiple @ mentions in a single request if multiple servers are needed
Add autocomplete suggestions when typing @ to show available MCP servers

Environment
OpenCode TUI
All platforms