GOAL_MESSAGE_SCHEMA_VERSION: Final = 5Canonical goal-message schema version.
Bump this whenever notice content changes in a way that makes an already
checkpointed notice misleading rather than merely stale. goal_state_notice_info
rejects any other version, so a resumed thread's outdated notice stops counting
as authoritative and the next model boundary appends a current one. Version 2
dropped the get_goal/get_rubric references version 1 notices carried, and
version 3 stopped truncating the only model-visible objective and rubric text.
Version 4 rejects oversized new state and supersedes any legacy oversized notice
with bounded recovery guidance. Version 5 counts HTML-escaped embedded text in
that budget, so version 4 notices with escape-heavy text are superseded.