The previous implementation used `pkill -P pid` which only kills direct
children. When Claude spawns subprocesses (like node workers), those
grandchild processes would become orphaned (PPID=1) when their parent
was killed, causing them to survive `gt shutdown -fa`.
The fix recursively finds all descendant processes and kills them in
deepest-first order, ensuring no process becomes orphaned during
shutdown.
Fixes: gt-wd3ce
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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