`detect_crashed_workers` calls `_pid_alive` on every `running` task whose
claim is held by this host. The check can transiently return False for a
freshly-spawned worker (fork → /proc-visibility lag, or reap-race
between SIGCHLD and parent reaping). When a second dispatcher ticks
inside that window it reclaims the task and spawns a duplicate worker.
Add `DEFAULT_CRASH_GRACE_SECONDS = 30` and an
`HERMES_KANBAN_CRASH_GRACE_SECONDS` env-var override.
`detect_crashed_workers` skips the liveness check when
`time.time() - started_at < grace`. The existing 15-minute claim TTL
still reclaims genuinely-crashed workers; grace only suppresses the
launch-window false positive.
`HERMES_KANBAN_CRASH_GRACE_SECONDS=0` is set on the `kanban_home`
fixture in `test_kanban_core_functionality.py` so existing tests that
assert immediate reclaim retain pre-fix semantics.
Companion to merged PR #23442 (`release_stale_claims`, closes#23025),
which addressed the same multi-dispatcher race in the stale-claim path.
Related: #20015 (`_pid_alive` false-negative behaviour),
c002668ff0