fix(cli): empty query emits error.v1 invalid_input for search + ask (Bug #14)

이전: `kebab search "" --json` / `kebab search "  " --json` / `kebab ask "" --json`
모두 exit=0 + silent 0 hit (search) 또는 LLM 빈 prompt round-trip (ask). user
mistake (typo, shell expansion 실수) 가 silent → debugging 비용.

이후: 양쪽 arm 에서 `query.trim().is_empty()` → kebab_app::StructuredError
(ErrorV1, code=invalid_input, hint 포함). exit=2 (StructuredError → 기존
exit_code() 의 generic non-zero path).

--bulk mode 는 영향 0 (bulk arm 이 query 무시).

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

crates/kebab-cli/tests/cli_empty_query.rs

@@ -0,0 +1,47 @@
+//! Integration tests for Bug #14: empty or whitespace-only query must emit
+//! error.v1 code=invalid_input and exit nonzero (not silent 0-hit return).
+
+use std::process::Command;
+use serde_json::Value;
+
+fn kebab_bin() -> String {
+    env!("CARGO_BIN_EXE_kebab").to_string()
+}
+
+fn parse_error_v1(stderr: &str) -> Value {
+    let last = stderr.lines().last().expect("expected error.v1 ndjson on stderr");
+    serde_json::from_str(last)
+        .unwrap_or_else(|e| panic!("expected ndjson on stderr: {e}\nstderr={stderr}"))
+}
+
+#[test]
+fn search_empty_query_emits_invalid_input() {
+    for q in ["", "   "] {
+        let out = Command::new(kebab_bin())
+            .args(["search", q, "--json"])
+            .output()
+            .expect("spawn kebab");
+        assert_ne!(
+            out.status.code(),
+            Some(0),
+            "empty/whitespace query must fail (q={q:?})"
+        );
+        let stderr = String::from_utf8_lossy(&out.stderr);
+        let v = parse_error_v1(&stderr);
+        assert_eq!(v["schema_version"], "error.v1", "stderr={stderr}");
+        assert_eq!(v["code"], "invalid_input", "stderr={stderr}");
+    }
+}
+
+#[test]
+fn ask_empty_query_emits_invalid_input() {
+    let out = Command::new(kebab_bin())
+        .args(["ask", "", "--json"])
+        .output()
+        .expect("spawn kebab");
+    assert_ne!(out.status.code(), Some(0));
+    let stderr = String::from_utf8_lossy(&out.stderr);
+    let v = parse_error_v1(&stderr);
+    assert_eq!(v["schema_version"], "error.v1");
+    assert_eq!(v["code"], "invalid_input");
+}