test(v0.17.0/A5): CLI hint surface e2e coverage (worker-1 nit)

PR #159 worker-1 review 의 LOW 가독성 nit 반영 — CLI stderr [hint]
line + --json hint shape 통합 test 가 없었음.

- search_plain_emits_short_query_hint_to_stderr — 빈 KB + 2자 query
  → stderr 가 "[hint]" + "3자 이상" 포함 확인.
- search_json_emits_hint_field_for_short_query — 동일 입력 --json
  → search_response.v1.hint 필드 set + 표준 advisory 문자열 정합.
- search_json_omits_hint_field_when_query_is_long_enough — 3자
  query → hint 필드 absent (additive serializer 의 None 제외 동작).

wire_search_response 5 → 8 PASS.

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

crates/kebab-cli/tests/wire_search_response.rs

@@ -243,3 +243,76 @@ fn search_plain_emits_truncated_hint_to_stderr() {
         "stderr must carry truncated hint: {stderr:?}"
     );
 }
+
+#[test]
+fn search_plain_emits_short_query_hint_to_stderr() {
+    // v0.17.0 A5 Step 6: 2-char query under trigram tokenizer emits
+    // empty hits + stderr `[hint]` advisory. Empty workspace is enough
+    // — hits are always empty so the hint condition depends only on
+    // query length (<3 chars trimmed) + non-raw mode + hits.is_empty.
+    let dir = tempfile::tempdir().unwrap();
+    let (cfg, workspace, _data) = common::write_config(dir.path(), 30);
+    common::ingest(&cfg, &workspace);
+
+    let (_stdout, stderr) = common::run_search_with_args(
+        &cfg,
+        &["--mode", "lexical", "ab"],
+    );
+    assert!(
+        stderr.contains("[hint]"),
+        "stderr must carry short-query hint: {stderr:?}"
+    );
+    assert!(
+        stderr.contains("3자 이상"),
+        "hint message must mention '3자 이상' (Korean advisory): {stderr:?}"
+    );
+}
+
+#[test]
+fn search_json_emits_hint_field_for_short_query() {
+    // v0.17.0 A5 Step 6: --json mode carries the same advisory on the
+    // `search_response.v1.hint` additive field. Empty hits + 2-char
+    // query + non-raw mode trips the helper. Verifies the MCP-visible
+    // surface (agents read the field instead of parsing stderr).
+    let dir = tempfile::tempdir().unwrap();
+    let (cfg, workspace, _data) = common::write_config(dir.path(), 30);
+    common::ingest(&cfg, &workspace);
+
+    let (stdout, _stderr) = common::run_search_with_args(
+        &cfg,
+        &["--json", "--mode", "lexical", "ab"],
+    );
+    let v: Value = serde_json::from_str(stdout.trim())
+        .unwrap_or_else(|e| panic!("not JSON: {stdout:?}: {e}"));
+    assert!(
+        v["hits"].as_array().unwrap().is_empty(),
+        "empty hits expected for short query in empty KB: {v}"
+    );
+    assert_eq!(
+        v["hint"].as_str().expect("hint field set on short empty result"),
+        "3자 이상 키워드 권장 (trigram tokenizer 제약)",
+        "hint must carry the standard advisory: {v}"
+    );
+}
+
+#[test]
+fn search_json_omits_hint_field_when_query_is_long_enough() {
+    // v0.17.0 A5 Step 6 (negative case): 3+ char query never trips
+    // hint, even on an empty KB. Verifies `serialize_search_response`
+    // omits the additive `hint` field when `None` so existing wire
+    // consumers stay backward-compatible.
+    let dir = tempfile::tempdir().unwrap();
+    let (cfg, workspace, _data) = common::write_config(dir.path(), 30);
+    common::ingest(&cfg, &workspace);
+
+    let (stdout, _stderr) = common::run_search_with_args(
+        &cfg,
+        &["--json", "--mode", "lexical", "abc"],
+    );
+    let v: Value = serde_json::from_str(stdout.trim())
+        .unwrap_or_else(|e| panic!("not JSON: {stdout:?}: {e}"));
+    assert!(
+        v.get("hint").is_none(),
+        "hint must be absent for ≥3-char queries: {v}"
+    );
+}