[copyberry]

Harden local MCP server process tree cleanup

Why

A local stdio MCP server can exit while its descendants remain alive. Cleaning up
only the server process can therefore leak processes after client shutdown.

What changed

• Contain locally launched MCP servers in non-breakaway Windows job objects and
  terminate the job during shutdown so descendants exit with the server.
• Preserve compatibility when job creation or assignment is unavailable by
  retrying without containment and terminating an owned process handle instead.
• Use the process-group member fallback when terminating piped processes on
  macOS.

Testing

• Cover descendant cleanup and breakaway prevention for both local MCP protocol
  modes on Windows.
• Cover the Windows nested-job assignment fallback and verify the suspended
  process resumes before handle-based termination.

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