fix(process): drain pipes in poll-based graceful-timeout to prevent deadlock#220
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The poll-based branch of run_with_graceful_timeout (taken when stop_event is non-None) never read from stdout/stderr pipes between polls. When oss-crs runs headless it prints all docker buildx bake output line-by-line; the runner-image build produces >1 MB of --progress rawjson, filling the OS pipe buffer (1 MB on Linux). The subprocess then blocks on pipe_write while the poll loop sleeps, and the run container never starts.
Fix: start background reader threads that drain stdout/stderr throughout the poll loop. On timeout or early-stop, send SIGTERM/SIGKILL via _signal_and_wait (which uses process.wait, not process.communicate) so there is no race with the background threads on the pipe FDs.