feat(v0.17.0/A5): trigram-aware build_match_string + SearchResponse.hint

PR-A 본체. plan Task A4 Step 1c + A5.

- lexical.rs::build_match_string 재설계: whole-phrase + token-AND
  OR-combined, 3자 미만 토큰 drop, 후보 없음 시 None (빈 MATCH
  회피). raw single-quote mode 유지.
- SearchResponse.hint additive — empty result + trimmed < 3 chars
  + non-raw 케이스에 short_query_hint helper 가 set.
- CLI 'kebab search' 가 [hint] stderr 한 줄 (text mode).
- TUI SearchState.short_query_hint + poll_worker stale-aware set
  + fire_search/mark_input_changed reset + dynamic_status 표시.
- docs/wire-schema/v1/search_response.schema.json hint additive.
- 신규 unit tests (lexical 9 PASS, 기존 2 expectation 갱신) +
  통합 회귀 (search_korean: multi_token + mixed, 3 PASS) +
  BM25 snapshot regen (trigram token stream).

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

crates/kebab-cli/src/wire.rs

@@ -92,6 +92,14 @@ pub fn wire_search_response(r: &kebab_app::SearchResponse) -> Value {
             map.insert("trace".to_string(), trace_v);
         }
     }
+    // v0.17.0 A5 Step 4b: emit `hint` only when set. Keeps responses
+    // that don't carry a hint backward-compatible with v0 consumers
+    // that don't know the field.
+    if let Some(hint) = &r.hint {
+        if let Value::Object(ref mut map) = v {
+            map.insert("hint".to_string(), Value::String(hint.clone()));
+        }
+    }
     tag_object(v, "search_response.v1")
 }
 
@@ -292,6 +300,7 @@ mod tests {
             next_cursor: Some("opaque-cursor-abc".to_string()),
             truncated: true,
             trace: None,
+            hint: None,
         };
         let v = wire_search_response(&r);
         assert_eq!(schema_of(&v), Some("search_response.v1"));
@@ -405,6 +414,7 @@ mod tests {
                 }],
                 timing: TraceTiming { lexical_ms: 5, vector_ms: 0, fusion_ms: 1, total_ms: 7 },
             }),
+            hint: None,
         };
         let v = wire_search_response(&r);
         assert_eq!(schema_of(&v), Some("search_response.v1"));
@@ -420,6 +430,7 @@ mod tests {
             next_cursor: None,
             truncated: false,
             trace: None,
+            hint: None,
         };
         let v = wire_search_response(&r);
         assert!(v.get("trace").is_none(), "trace field absent when None");