feature/IAV-1323 : feat(memory): Phase 2 checkpoint, soft trim, and rolling summary refinements
- Add ConversationSession.memoryCheckpointInboundUid; load verbatim history only after checkpoint (BaseAgentMessageRepository.findInboundMessagesWithIdAfter).
- Introduce MemoryInboundWindowLoader shared by getMemoryContext and ConversationSummaryGenerator.
- Trim serialized history to effective_history_cap * history-soft-cap-ratio (default 0.5) for headroom; document iavia.memory.history-soft-cap-ratio.
- Fold oldest half of inbound exchanges on summary; advance checkpoint after LLM merge; default summary-trigger-ratio to 1.0 (configurable).