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/src/main.rs

@@ -819,6 +819,17 @@ fn run(cli: &Cli) -> anyhow::Result<()> {
 
             // p9-fb-42: bulk mode requires no query; single-query mode requires query.
             let query_text = match query.as_ref() {
+                Some(q) if q.trim().is_empty() => {
+                    return Err(anyhow::Error::new(kebab_app::StructuredError(
+                        kebab_app::ErrorV1 {
+                            schema_version: kebab_app::ERROR_V1_ID.to_string(),
+                            code: "invalid_input".to_string(),
+                            message: "query is empty; provide a non-empty search term or use --bulk".into(),
+                            details: serde_json::Value::Null,
+                            hint: Some("e.g. `kebab search 'rust async'` or `kebab search --bulk < queries.ndjson`".into()),
+                        },
+                    )));
+                }
                 Some(q) => q.clone(),
                 None => {
                     return Err(anyhow::anyhow!("query is required unless --bulk is set"));
@@ -988,6 +999,17 @@ fn run(cli: &Cli) -> anyhow::Result<()> {
             multi_hop,
         } => {
             let cfg = kebab_config::Config::load(cli.config.as_deref())?;
+            if query.trim().is_empty() {
+                return Err(anyhow::Error::new(kebab_app::StructuredError(
+                    kebab_app::ErrorV1 {
+                        schema_version: kebab_app::ERROR_V1_ID.to_string(),
+                        code: "invalid_input".to_string(),
+                        message: "query is empty; provide a non-empty prompt".into(),
+                        details: serde_json::Value::Null,
+                        hint: Some("e.g. `kebab ask \"explain this code\"`".into()),
+                    },
+                )));
+            }
             if *stream {
                 // p9-fb-33: streaming branch. Background thread runs
                 // ask_with_config (which calls into the rag pipeline);