MCP server processes not killed on CLI exit

### Describe the bug

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### Affected version

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### 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

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### Additional context

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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.

Written by the indexing model from the issue text.