MCP server processes not killed on CLI exit

### Describe the bug

_No response_

### Affected version

_No response_

### Steps to reproduce the behavior

Body: After exiting copilot, ssh-mcp (and other MCP) child processes remain alive in Activity Monitor. Each
consumes ~tens of MB. Expected behavior: CLI should send SIGTERM to all spawned MCP child processes on exit.
Steps to reproduce: 1. Start copilot with SSH MCP servers configured. 2. Exit with Ctrl+D or /exit. 3. Run
pgrep -la mcp — processes still alive.

### Expected behavior

_No response_

### Additional context

_No response_

Start by reproducing the issue from the copilot CLI with an SSH MCP server configured, exiting via Ctrl+D and /exit, then check the remaining processes with `pgrep -la mcp`. Trace the CLI entry point's shutdown and MCP child-process handling; done means all spawned MCP processes receive SIGTERM and no longer remain after exit.

由索引模型根據 Issue 內容生成。