A budget-aborted keep-alive subagent's job row (job id == agent id) settles
failed and is retained ~5 min; executeCancel short-circuited to
already_completed for that window, leaving the zombie registration
unkillable exactly when the user wants it dead. Fall through to
cancelAgentRegistration for settled rows; keep already_completed when no
lingering registration exists.
Also stop wiring AgentLifecycleManager.global() onto SDK sessions created
with a caller-supplied agentRegistry: the global lifecycle releases through
AgentRegistry.global(), so it would report a cancel while releasing an
unrelated global ref. Without a lifecycle, cancel falls back to
dispose + unregister on the session's own registry.
Addresses both Codex P2 review findings on #6319.
can1357
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