Compaction now decides whether to strip thinking blocks via a 'wise comet' policy

Compaction now uses a new helper to decide, per-compaction, whether trailing thinking blocks in the kept tail should be stripped, based on the resolved model and thinking type. It logs the decision, whether from an env override, the thinking_type default, or a flag, and marks the boundary with a thinking_stripped marker when stripping occurs.

During reactive compaction, preserved messages now pass through a new step that may strip thinking blocks and insert a marker attachment, with new telemetry fields tracking how many thinking blocks were kept versus stripped and what determined the decision.