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-cli/tests/cli_error_wire.rs

@@ -2,11 +2,10 @@
 //! on stderr while non-json mode emits the legacy `error:` text prefix.
 //!
 //! The `config_invalid` code is triggered by supplying an *existing* but
-//! malformed TOML file via `--config`. Note: supplying a *non-existent*
-//! path does NOT trigger this error — Config::load silently falls back to
-//! defaults when the specified config file is absent (by design, so that
-//! `kebab doctor` runs before `kebab init` is ever called). A file that
-//! exists but fails TOML parsing is the reliable path to `config_invalid`.
+//! malformed TOML file via `--config`. A file that exists but fails TOML
+//! parsing is the reliable path to `config_invalid`. Supplying a path that
+//! does not exist emits `config_not_found` instead (Bug #10 fix, v0.20.0
+//! bugfix3); see `cli_config_not_found.rs` for those tests.
 
 use std::process::Command;