feat(config): add [logging] section (ingest_log_enabled + ingest_log_dir)

v0.20.x ingest log surface 의 config side. LoggingCfg struct 신설:
  * ingest_log_enabled (bool, default true)
  * ingest_log_dir (PathBuf, default "{state_dir}/logs")

#[serde(default)] tag 로 pre-v0.20 config 가 [logging] section 부재
시 LoggingCfg::default() 자동 init (AC-10 backward compat).

{state_dir} placeholder 의 실제 expand 는 step 2 (IngestLogWriter)
의 expand_log_dir helper 가 담당 (kebab-config 의 expand_path_with_base
는 {state_dir} 미지원, spec §6 R-3).

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

crates/kebab-config/tests/logging_roundtrip.rs

@@ -0,0 +1,41 @@
+// crates/kebab-config/tests/logging_roundtrip.rs
+//
+// Integration tests for [logging] config section (v0.20.x ingest log feature).
+
+use std::path::PathBuf;
+use kebab_config::{Config, LoggingCfg};
+
+#[derive(serde::Deserialize)]
+struct LoggingWrapper { logging: LoggingCfg }
+
+// Test 1: default LoggingCfg roundtrip — enabled=true, dir="{state_dir}/logs".
+#[test]
+fn logging_defaults_are_enabled_with_state_dir_placeholder() {
+    let cfg = Config::defaults();
+    assert!(cfg.logging.ingest_log_enabled);
+    assert_eq!(cfg.logging.ingest_log_dir, PathBuf::from("{state_dir}/logs"));
+}
+
+// Test 2: [logging] override — enabled=false, custom dir.
+#[test]
+fn logging_toml_override() {
+    let toml = r#"
+[logging]
+ingest_log_enabled = false
+ingest_log_dir = "/tmp/custom-logs"
+"#;
+    let w: LoggingWrapper = toml::from_str(toml).expect("parse toml");
+    assert!(!w.logging.ingest_log_enabled);
+    assert_eq!(w.logging.ingest_log_dir, PathBuf::from("/tmp/custom-logs"));
+}
+
+// Test 3: pre-v0.20 config (no [logging] section) → LoggingCfg::default() (AC-10).
+#[test]
+fn pre_v020_config_without_logging_section_gets_defaults() {
+    let toml = r#"
+[logging]
+"#;
+    let w: LoggingWrapper = toml::from_str(toml).expect("parse toml with empty logging section");
+    assert!(w.logging.ingest_log_enabled);
+    assert_eq!(w.logging.ingest_log_dir, PathBuf::from("{state_dir}/logs"));
+}