Whether concurrent active tasks are legitimate depends on runtime
concurrency the tool cannot observe, but whether a deployment's agents
ever fan out is knowable at composition time. `allowParallelInProgress`
(default true) therefore replaces the hardcoded policy: the flag moves
the model-facing instruction and the accepted input together, so a
deployment running strictly sequential agents can restore the
single-active discipline from cordis.yml.
The durable-log invariant does not follow the flag. A log written while
parallel work was allowed must still replay after a deployment tightens
the policy, so the invariant stays silent on the active count.
