Compaction was reporting only 189 tokens removed for 6 messages because
Phase 1 (tool result pruning) hollowed out messages before Phase 2
(summary compaction) measured them. Now captures pre-pruning token count
and reports combined savings from both phases.
Also threads RunLog through SessionManager to emit tool_result_pruning
and compaction_detail events, and adds preflight pruning stats logging.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>