feat(rag): fb-41 PR-2 — RagPipeline::ask_multi_hop skeleton (fixed depth=2)

PR-2 of fb-41 multi-hop RAG. Decompose + retrieve + synthesize 3-stage
pipeline가 `opts.multi_hop=true` 일 때 dispatch. Dynamic decide loop
는 PR-3.

- `AskOpts.multi_hop: bool` 필드 추가 + `impl Default for AskOpts`
  도입 (HOTFIXES 2026-05-07 의 known limitation 해소). 9 explicit
  init site 모두 `multi_hop: false` 추가 — Default 도입으로 향후
  `..Default::default()` 점진 migrate 가능.
- `RagPipeline::ask` 의 entry 에 dispatcher 한 줄
  (`if opts.multi_hop { return self.ask_multi_hop(...) }`).
- `RagPipeline::ask_multi_hop` 신규 method. 1) decompose LLM call
  → JSON array of strings parse, 2) 각 sub-query 로 retrieve +
  chunk_id dedup pool, 3) score gate / no-chunks 가드, 4)
  pack_context (single-pass 와 helper 공유), 5) synthesize LLM
  call w/ MULTI_HOP_SYNTHESIZE_SYSTEM_PROMPT, 6) citation extract
  + Answer build. `prompt_template_version` = "rag-multi-hop-v1"
  로 stamp — eval `compare` 가 single-pass vs multi-hop 분리.
- Prompt const 신규: MULTI_HOP_DECOMPOSE_SYSTEM_PROMPT +
  MULTI_HOP_DECOMPOSE_USER_TEMPLATE + MULTI_HOP_SYNTHESIZE_SYSTEM_PROMPT
  + PROMPT_TEMPLATE_VERSION_MULTI_HOP + MULTI_HOP_MAX_SUB_QUERIES_DEFAULT.
- `kebab_core::RefusalReason::MultiHopDecomposeFailed` variant 신규.
  Cascade: kebab-store-sqlite `refusal_reason_label` + kebab-tui `ask
  refusal render` exhaustive match 갱신.
- `parse_decompose_response` + `strip_markdown_json_fence` helper —
  markdown code fence (```json / ```) strip + JSON array of strings
  parse + trim + drop empty + cap at MULTI_HOP_MAX_SUB_QUERIES_DEFAULT.
  None 반환 시 caller 가 `MultiHopDecomposeFailed` refusal.

Tests (55 passing total, 8 신규):
- 6 unit (parse_decompose_response 의 bare array / fence variants /
  garbage / cap / trim 회귀 핀).
- 2 integration: `ask_multi_hop_dispatches_and_decompose_garbage_refuses`
  (decompose garbage → MultiHopDecomposeFailed + 정확히 1 LLM call) +
  `ask_with_multi_hop_false_keeps_single_pass_path` (회귀 핀, 기존
  caller 자동 backwards-compat).

Happy-path multi-hop (decompose 성공 → synthesize) 의 integration
test 는 ScriptedLm helper 가 PR-3 의 decide loop 와 함께 도입될
때 같이 추가. 현 `MockLanguageModel` 는 canned single response 라
2-LLM-call sequence 핀 불가.

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

crates/kebab-eval/src/runner.rs

@@ -179,6 +179,10 @@ fn execute_query(app: &App, gq: &GoldenQuery, opts: &EvalRunOpts) -> QueryResult
             history: Vec::new(),
             conversation_id: None,
             turn_index: None,
+            // p9-fb-41: golden eval baseline runs are single-pass; the
+            // multi-hop path is opted into per query via a future
+            // fixture flag (PR-4+) once the runner learns to dispatch.
+            multi_hop: false,
         };
         match app.ask(&gq.query, ask_opts) {
             Ok(ans) => Some(ans),