chore(rag): PR #167 회차 1 리뷰 반영

- `parse_decompose_response_drops_partial_empty_keeps_valid` 신규
  회귀 핀 — `["", "valid q", "  "]` → `["valid q"]` (trim+filter
  chain 동작 pin).
- `multi_hop_decompose` 의 `stop: Vec::new()` 옆 doc comment 추가 —
  의도 명시 (instruction-following 모델 기대 + prose 추가 시
  MultiHopDecomposeFailed refusal 가 policy). 회차 1 question 의
  답변.
- plan 의 PR-3 implementation order 에 회차 1 carry-over 추가:
  1) ask + ask_multi_hop 의 §4-§9 mirror → 공통 helper 추출,
  2) decompose template 의 substitution corner case → format!
     named arg 으로 교체.

회차 1 의 다른 suggestion (mirror refactor, substitution corner
case, history block helper) 는 PR-3 합리적 timing 으로 plan 에
명시 — 회차 2 reply 에 정리.

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

crates/kebab-rag/src/pipeline.rs

@@ -908,6 +908,16 @@ impl RagPipeline {
         let req = GenerateRequest {
             system: MULTI_HOP_DECOMPOSE_SYSTEM_PROMPT.to_string(),
             user,
+            // Empty stop is intentional. Instruction-following models
+            // (gemma3:4b+ / gemma4:e4b / Claude / GPT-4) honor the
+            // "JSON array only" prompt rule, so prose past the
+            // closing `]` is rare. If a downstream LM does append
+            // prose, `parse_decompose_response` returns `None` and
+            // the caller surfaces `MultiHopDecomposeFailed` — that
+            // is the policy. Adding a trailing-`]` stop sequence
+            // risks truncating the array (LM emits the close bracket
+            // and we cut the response one token too early), which
+            // is a worse failure mode than the explicit refusal.
             stop: Vec::new(),
             // JSON array of up to 5 sub-questions is short. 512 is a
             // comfortable cap that fits in any context window without
@@ -1495,6 +1505,16 @@ mod tests {
         assert_eq!(out, vec!["rust async", "tokio runtime"]);
     }
 
+    /// Partial-empty case — drop the empties, keep the rest. Pins
+    /// the trim-then-filter chain in `parse_decompose_response` so a
+    /// future refactor that reorders the steps (e.g. take-then-trim)
+    /// can't accidentally swallow valid sub-queries.
+    #[test]
+    fn parse_decompose_response_drops_partial_empty_keeps_valid() {
+        let out = parse_decompose_response(r#"["", "valid q", "  "]"#).unwrap();
+        assert_eq!(out, vec!["valid q"]);
+    }
+
     #[test]
     fn est_tokens_approx_quarters() {
         assert_eq!(est_tokens(""), 0);