feat(rag): multi-turn ask — Turn struct + ask_with_history + token budget (p9-fb-15)

Spec PR #59 의 §3.8 multi-turn behaviour 구현. RAG facade 가 prior
turns 받아 prompt 에 prepend, retrieval query expansion 적용,
Answer 에 conversation_id / turn_index 채움.

신규 (kebab-core):
- Answer 에 conversation_id (Option<String>) / turn_index (Option<u32>)
  field 추가. serde skip_serializing_if 로 single-shot 의 wire
  output 변경 0 (기존 외부 wrapper 영향 없음).
- Turn struct (question + answer + citations + created_at).
- RefusalReason::LlmStreamAborted variant.

신규 (kebab-rag):
- AskOpts 에 history (Vec<Turn>) / conversation_id / turn_index 3 field.
- AskOpts::single_shot(mode) helper.
- RagPipeline::ask_with_history(query, history, conversation_id,
  turn_index, opts) — combined opts 로 ask 호출.
- expand_query_with_history: history.last() 의 answer 첫 200 자
  concat 해 SearchQuery.text 확장 (spec §3.8 의 \"cheap concat\";
  LLM-based standalone-question rewriting 은 P+).
- serialize_history + remaining_history_budget_chars: spec 의 priority
  enforcement — system+question 필수, retrieved chunks 가 차지한
  뒤 남은 char budget 안에서 newest 우선, oldest drop.
- ask 본문: history 가 비어있지 않으면 [이전 대화] 블록을 user
  prompt 위에 prepend. Answer 생성 site 3 곳 (정상 / NoChunks /
  ScoreGate refuse) 모두 conversation_id / turn_index 채움.

신규 (kebab-store-sqlite):
- refusal_reason_label 가 LlmStreamAborted → 'llm_stream_aborted'.

기존 caller 변경 (single-shot 동작 동일):
- kebab-cli main.rs Cmd::Ask: AskOpts 에 history=Vec::new(),
  conversation_id=None, turn_index=None 명시 (CLI multi-turn 은
  p9-fb-18 의 --session/--repl 가 채움).
- kebab-tui src/ask.rs spawn site 동일 (multi-turn UI 는 p9-fb-16).
- kebab-eval runner.rs golden eval 동일 (single-shot per query).
- kebab-app tests/ask_smoke.rs / kebab-tui tests/ask.rs / kebab-rag
  tests/pipeline.rs / kebab-eval metrics.rs Answer literal 갱신.

Test:
- 9 신규 lib unit (expand_query 4 / serialize_history 3 / remaining_budget 2).
- 기존 12 PASS 회귀 0.

Plan 갱신:
- p9-fb-15 status planned → in_progress. 머지 후 한 줄 commit
  으로 completed flip.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/kebab-store-sqlite/src/answers.rs

@@ -98,6 +98,7 @@ fn refusal_reason_label(r: &RefusalReason) -> &'static str {
         RefusalReason::LlmSelfJudge => "llm_self_judge",
         RefusalReason::NoIndex => "no_index",
         RefusalReason::NoChunks => "no_chunks",
+        RefusalReason::LlmStreamAborted => "llm_stream_aborted",
     }
 }