fix(kanban): detect darwin zombie workers

[LeonSGP43]

Fixes #20015

Summary

• extend Kanban _pid_alive() zombie detection to macOS/Darwin using ps -o stat= after kill(pid, 0) succeeds
• treat Darwin process states containing Z as dead so crash detection can reclaim defunct workers promptly
• keep the existing Linux /proc/<pid>/status path unchanged

Scope

This is limited to Kanban dispatcher worker liveness checks. It does not change gateway scoped locks or process spawning semantics.

Verification

• scripts/run_tests.sh tests/hermes_cli/test_kanban_core_functionality.py::test_pid_alive_helper tests/hermes_cli/test_kanban_core_functionality.py::test_pid_alive_detects_darwin_zombie tests/hermes_cli/test_kanban_core_functionality.py::test_detect_crashed_workers_reclaims -> 3 passed
• git diff --check

[teknium1]

Merged via PR #20188 (commit 1a03e3b). Your commit was cherry-picked onto current main with your authorship preserved in git log. Clean fix — ps -o stat= is exactly the right tool on Darwin, and the defensive fallback-to-kill(0)-answer on probe failure is the right call. Thanks.

#20188