`_kill_tree` SIGKILLs the per-file pytest process group, which misses any
child that left the group with `start_new_session=True` (an auto-started
`gateway run`, a setsid server). Those are reparented to init and run
forever: one host accumulated 215 orphan gateways (~52 GB RSS, up to 21 h
old), and every `hermes update` listed each as "another Hermes home — left
alone".
After every attempt the runner now SIGKILLs any process whose environment
still points into that attempt's unique temp root (PYTEST_DEBUG_TEMPROOT /
TMPDIR, or a HOME / HERMES_HOME a fixture created under it). The root is
unique per attempt, so the match names only that attempt's processes.
Linux-only (/proc); the process-group kill stays the fallback elsewhere.
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