U5: action-gating contexts, the heartbeat deferral gate, a two-pass
resumeTaskForAgent, and the reverse-direction agent.taskId guards (via a
new isAgentEffectivelyExecuting callback wired at the in-process runtime)
all consult the column-effective agent; the restart watcher re-resolves
column bindings per tick for bound graph sessions, hot-swapping on
agent-changed and falling back without restart on agent-deleted.
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