A capability that runs a compactor on before_model_request, shrinking the canonical history (and the request about to be sent) when it grows too large.

Add it to an agent's :capabilities:

%ExAgent.Compaction.Capability{
  compactor: ExAgent.Compaction.Summary,
  opts: [threshold_tokens: 6000, keep_recent: 8, summarize: &my_summarize/1]
}