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/src/lib.rs

@@ -64,6 +64,11 @@ pub struct Config {
     /// built-in defaults (OCR disabled — opt-in for scanned PDF KB).
     #[serde(default = "PdfCfg::defaults")]
     pub pdf: PdfCfg,
+    /// v0.20.x ingest log surface. `#[serde(default)]` so pre-v0.20
+    /// config files without a `[logging]` section load with built-in
+    /// defaults (enabled=true, dir=~/.local/state/kebab/logs).
+    #[serde(default)]
+    pub logging: LoggingCfg,
     /// p9-fb-05: directory of the on-disk config file this `Config`
     /// was loaded from, if any. Populated by `Config::from_file` /
     /// `Config::load` — never serialized (`#[serde(skip)]`). Used by
@@ -423,6 +428,36 @@ impl Default for PdfCfg {
     fn default() -> Self { Self::defaults() }
 }
 
+/// v0.20.x ingest log surface: structured ndjson log written per ingest run.
+/// `#[serde(default)]` on Config.logging ensures pre-v0.20 config files load cleanly.
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct LoggingCfg {
+    /// Write structured ndjson log for each ingest run. Default `true`.
+    /// Set `false` to suppress log file creation entirely (AC-6).
+    #[serde(default = "default_ingest_log_enabled")]
+    pub ingest_log_enabled: bool,
+
+    /// Directory for per-run log files. Default `{state_dir}/logs`.
+    /// `{state_dir}` expands to the XDG state dir (e.g. `~/.local/state/kebab`).
+    /// Log file accumulation is user-managed — no rotation policy (spec §6 R-1).
+    #[serde(default = "default_ingest_log_dir")]
+    pub ingest_log_dir: PathBuf,
+}
+
+fn default_ingest_log_enabled() -> bool { true }
+fn default_ingest_log_dir() -> PathBuf {
+    PathBuf::from("{state_dir}/logs")
+}
+
+impl Default for LoggingCfg {
+    fn default() -> Self {
+        Self {
+            ingest_log_enabled: default_ingest_log_enabled(),
+            ingest_log_dir: default_ingest_log_dir(),
+        }
+    }
+}
+
 /// v0.20.0 sub-item 1: scanned PDF OCR via Ollama vision LLM. Default
 /// disabled — opt-in because OCR adds ~45-100s per scanned page on CPU
 /// (qwen2.5vl:3b, remote). Enable for book / paper scan KB.
@@ -649,6 +684,7 @@ impl Config {
             ui: UiCfg::defaults(),
             ingest: IngestCfg::default(),
             pdf: PdfCfg::defaults(),
+            logging: LoggingCfg::default(),
             // p9-fb-05: defaults are not loaded from disk, so no
             // source_dir. Relative `workspace.root` (rare with
             // defaults) falls back to caller `cwd` via the

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"));
+}