post-compaction 추적과 대화별 사용량 계측 보강

- claude-code의 post-compaction 플래그 흐름을 참고해 compact 직후 첫 응답을 별도로 추적하고 대화 단위 후속 사용량을 집계함

- ChatConversation에 pending post-compaction, 응답 회수, prompt/completion token 누적 필드를 추가하고 AX Agent 컨텍스트 카드 hover에 표시함

- README 및 docs/DEVELOPMENT.md에 2026-04-04 23:47 (KST) 기준 이력을 반영함

- 검증: dotnet build src/AxCopilot/AxCopilot.csproj -c Release -v minimal -p:OutputPath=bin\\verify\\ -p:IntermediateOutputPath=obj\\verify\\ (경고 0 / 오류 0)

docs/DEVELOPMENT.md

@@ -2,6 +2,7 @@
 
 - Document update: 2026-04-04 23:47 (KST) - Added AX-side session-memory compaction, collapse/snip trimming, and per-conversation compaction usage tracking so compact now records staged reductions instead of only the last before/after snapshot.
 - Document update: 2026-04-04 23:47 (KST) - The AX Agent context tooltip now shows session totals for compaction count, saved tokens, session-memory passes, microcompact boundaries, and snipped messages for the current conversation.
+- Document update: 2026-04-04 23:47 (KST) - Added a post-compaction flag and conversation-level follow-up usage counters so AX can distinguish the first response after compaction and report its prompt/completion token cost separately.
 
 - Document update: 2026-04-04 23:32 (KST) - Upgraded ContextCondenser from a two-step path to a three-step path (tool-result truncate -> microcompact -> summarize) so long sessions can strip older execution/tool chatter before invoking the LLM summary stage.
 - Document update: 2026-04-04 23:32 (KST) - Added AX-side microcompact boundaries that compress older tool results, execution metadata, and oversized messages into lighter boundary summaries, bringing the compact flow closer to the staged approach used in claude-code.