The documentation claimed fallback activates 'at most once per session',
but the actual implementation restores the primary model at the start of
every run_conversation() call via _restore_primary_runtime().
Relevant source: run_agent.py lines 1666-1694 (snapshot), 6454-6517
(restore), 8681-8684 (called each turn).
Updated the One-Shot info box and the summary table to accurately
describe the per-turn restoration behavior.
9357db2844