Each chat's jobs live in <workspace>/.volcanic/scheduler/jobs.json. On start the
scheduler loads every chat's file, moves the retired shared
<agentDir>/scheduler/jobs.json into them and renames it .migrated-<ts>.
A VolcanicSecretary jobs.json (bare array) in a chat's folder is converted
in place for that chat; the original is kept as .secretary-<ts>.
Also adds the VolcanicSecretary data migration plan.