Implements the per-intent rotation shape from #85: a task created with
llm_rotate=true distributes worker intents round-robin over its LLM chain
heads, so lane A can hunt on one model while lane B hunts on another
(different lenses per intent, no extra tokens). Default stays the classic
ordered failover from the active head; planner/goals/mainagent keep the
active head always.
Mechanics: the engine pins the next chain head into the run context when a
worker claims an intent; the pin travels through the provider resolver
(hooks.current now takes ctx). A pin is only a dispatch preference - it must
belong to the task chain and never bypasses failover state (an exhausted
chain stays exhausted). Schema: tasks.llm_rotate boolean, default false,
idempotent ALTER; exposed on the create payload and the task DTO.
Closes#85 (the ensemble/P4 shape stays parked as before)
