fix(kanban): detect darwin zombie workers (salvages #20023)

[teknium1]

Kanban dispatcher now correctly detects zombie worker processes on macOS, so crashed workers get reclaimed on the next tick instead of tying up their task for the full claim_expires window (~15 min).

Salvaged from #20023 (@LeonSGP43).

Root cause: _pid_alive() used os.kill(pid, 0) which succeeds for zombie processes because the process table entry still exists post-exit, pre-reap. On Linux it fell through to /proc/<pid>/status to read State: Z, but on macOS there's no /proc, so the zombie check was a documented no-op. A worker that crashed at startup (missing skill, bad credential, import error) stayed "alive" to the dispatcher until claim TTL expired, creating a ~5 min dispatcher cadence × 15 min TTL = 3+ wasted re-spawn attempts per stuck task, all of which crashed identically.

Changes

• hermes_cli/kanban_db.py: after kill(pid, 0) succeeds on Darwin, shell out to ps -o stat= -p <pid> with a 1s timeout. Return False if ps exits non-zero (no such process) or if the BSD stat field contains Z. If the probe itself errors, keep the optimistic kill(0) answer — conservative default.
• tests/hermes_cli/test_kanban_core_functionality.py: new test_pid_alive_detects_darwin_zombie covering the Darwin branch with a mocked ps returning Z+.
• Linux /proc/<pid>/status path unchanged.

Validation

	Before	After
macOS: worker crashes at startup	_pid_alive → True for ~15 min until claim_expires; dispatcher re-spawns 3+ times	_pid_alive → False on next tick; task reclaimed via crashed-worker path
Linux: zombie worker	/proc/<pid>/status peek returns False (unchanged)	unchanged
Windows / other POSIX	no zombie check (unchanged)	unchanged
ps probe fails (unexpected)	N/A	falls back to kill(0) answer (optimistic)
Targeted tests	—	174/174 pass (test_kanban_core_functionality + test_kanban_db)

Closes #20015

Co-authored-by: LeonSGP43 cine.dreamer.one@gmail.com