File-based task store.
 Layout:
   {baseDir}/
     {sessionDir}/
       tasks.json            — index: all task records, ordered by Sequence
       tasks/
         {taskId}/
           meta.json         — one task's full record
           sessions/         — the sub-session(s) this task spawned (written by their grains)

 {baseDir} is the shared `sessions/` root. Paths resolve through `SessionPaths.sessionDir`
 so a task's metadata and the sub-session it starts nest together under tasks//,
 and everything for a session shares one parent folder.