review(회차1): AskOpts::single_shot helper 제거 (yagni)

회차 1 nit 반영. helper 가 본 PR 안 caller 0 — 모든 사용처가
struct literal 패턴. CLAUDE.md "Don't add abstractions beyond
what the task requires" 룰. 미래 caller 가 필요 시 본인이 추가.

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

crates/kebab-rag/src/pipeline.rs

@@ -98,25 +98,6 @@ pub struct AskOpts {
     pub turn_index: Option<u32>,
 }
 
-impl AskOpts {
-    /// Default knobs for a single-shot ask (no history, no
-    /// conversation_id). `k` falls through to the config floor in
-    /// `RagPipeline::ask`.
-    pub fn single_shot(mode: SearchMode) -> Self {
-        Self {
-            k: 0,
-            explain: false,
-            mode,
-            temperature: None,
-            seed: None,
-            stream_sink: None,
-            history: Vec::new(),
-            conversation_id: None,
-            turn_index: None,
-        }
-    }
-}
-
 // ── RagPipeline ─────────────────────────────────────────────────────────────
 
 /// Single-threaded RAG orchestrator. See module docs for the stage list.