Remove trend_phase context mode (v0.2.0)

[davidkfoss]

Summary

• Removes the trend_phase context mode and TREND_PHASE_CONTEXT_BUCKETS,
  matching the cleanup already done in the experiments repo.
• Drops total_training_steps from AEES and StructuredEpisodeManager
  (its only use was the trend_phase phase split).
• Collapses the three redundant fields on StructuredSelection
  (context_bucket_id, context_bucket_name, context_trend) into one
  context_bucket, and the corresponding three log keys into one
  context_buckets. Inlines _resolve_bucket_names and merges the two
  context-resolution helpers into one _resolve_context.
• Bumps version to 0.2.0 (breaking removal of public symbols).