Replace 17 parallel log lists with list[EpisodeSummary]

[davidkfoss]

Problem

StructuredEpisodeManager maintains 17 parallel lists (episode.py:149-168) that must stay synchronized for every episode. _append_structured_logs (episode.py:257-281) is essentially bookkeeping for that invariant — every time a new episode-level field is added, it must be threaded through here too, or it silently drops out of get_logs().

Proposal

Store list[EpisodeSummary] instead. EpisodeSummary already carries every one of those fields. Compute the per-key column views lazily inside get_logs().

Wins

• _append_structured_logs deletes entirely.
• The 17 list attributes collapse to one.
• A class of "I added a field to EpisodeSummary but forgot to log it" bugs disappears by construction.

Acceptance criteria

• episode.py stores _episodes: list[EpisodeSummary].
• get_logs() returns the same dict-of-lists shape it returns today (no API change for callers).
• _append_structured_logs removed.
• Existing tests pass without modification.