Task model fields (execution/planning/validator provider+modelId) are
snapshotted once at task-creation time from the workflow's model-lane
default and never re-synced. Changing a workflow's default (e.g. fixing
a stale model id) silently leaves already-created tasks pinned to the
old value with no visibility — this is exactly how 52 tasks stayed
pinned to a stale claude-sonnet-4-6 default after it was corrected.
Add TaskStore.getModelLaneDrift(workflowId, before, after), a read-only
diff over the three model lanes that lists non-terminal tasks still
pinned to a lane's old value. Wire it into
PATCH /workflows/:id/setting-values so the response includes
`modelDrift` whenever a lane change orphans existing tasks. Operators
can then act via the existing POST /tasks/batch-update-models.
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