run_command_with_timeout piped stdout/stderr but only read them after
the child exited. When test output exceeded the 64KB OS pipe buffer,
the child blocked on write() while the parent blocked on waitpid() —
a permanent deadlock that caused every merge pipeline to hang.
Drain both pipes in background threads so the buffers never fill.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
076324c470