fix(config): emit error.v1 code=config_not_found for missing --config path (Bug #10)

이전: `kebab search "rust" --config /tmp/nonexistent.toml --json` 가 exit=0 +
`{"hits":[]}` silent fallback to XDG default. typo / wrong path 가 0-hit 으로만
surface — debugging nightmare.

이후: kebab_config::ConfigNotFound thiserror::Error 추가, Config::load 의
`Some(p) if !p.exists()` arm 이 anyhow::Error::new(ConfigNotFound { path })
return. kebab_app::error_wire::classify 가 downcast → ErrorV1 code=config_not_found,
hint, details.path 채워서 stderr 에 ndjson 으로 emit.

R-1 (relative path): std::path::Path::exists() 는 cwd-relative — 별도 작업 없이
absolute + relative 모두 cover. integration test 두 개로 검증.

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

crates/kebab-config/src/lib.rs

@@ -24,6 +24,15 @@ pub struct ConfigInvalid {
     pub cause: String,
 }
 
+/// p20-bugfix3 Bug #10: explicit `--config <path>` was missing → silent
+/// fallback to defaults instead of fail-fast. `kebab-app::error_wire::classify`
+/// downcasts → `code: "config_not_found"` ErrorV1.
+#[derive(Debug, thiserror::Error)]
+#[error("config file does not exist: {path}")]
+pub struct ConfigNotFound {
+    pub path: PathBuf,
+}
+
 #[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
 pub struct Config {
     pub schema_version: u32,
@@ -688,7 +697,11 @@ impl Config {
     pub fn load(path: Option<&Path>) -> anyhow::Result<Self> {
         let from_disk = match path {
             Some(p) if p.exists() => Self::from_file(p)?,
-            Some(_) => Self::defaults(),
+            Some(p) => {
+                return Err(anyhow::Error::new(ConfigNotFound {
+                    path: p.to_path_buf(),
+                }));
+            }
             None => {
                 let p = Self::xdg_config_path();
                 if p.exists() {
@@ -1817,4 +1830,17 @@ mod fb27_tests {
             signal.cause
         );
     }
+
+    #[test]
+    fn config_load_explicit_nonexistent_path_returns_config_not_found() {
+        // Bug #10: --config /tmp/nonexistent.toml → silent fallback 금지.
+        let p = std::path::Path::new("/tmp/__kebab_bugfix3_nonexistent.toml");
+        assert!(!p.exists(), "test precondition: path must not exist");
+
+        let err = Config::load(Some(p)).expect_err("expected ConfigNotFound");
+        let signal = err
+            .downcast_ref::<ConfigNotFound>()
+            .expect("from_load error should downcast to ConfigNotFound");
+        assert_eq!(signal.path, p.to_path_buf());
+    }
 }