feat(app): log retention — keep_recent_runs + retention_days (Enhancement 4)

LoggingCfg gains two fields with serde defaults: keep_recent_runs
(default 100, top-N file retention) and retention_days (default 30,
time-based retention for both ndjson files and the SQLite mirror).

IngestLogWriter::open now runs cleanup_old_logs before creating a new
ingest-*.ndjson — delete iff (idx >= keep_recent) OR (modified <=
cutoff). ingest_with_config_opts also calls
SqliteStore::prune_pdf_ocr_events(retention_days) at ingest start so
the SQLite mirror tracks the same retention window.

Backward compat (AC-9): both new fields use #[serde(default = ...)],
so a pre-v0.20.x config with only [logging] ingest_log_enabled +
ingest_log_dir parses unchanged. kebab init writes the new defaults
automatically via Config::default() -> toml::to_string_pretty (AC-12).

docs/SMOKE.md config example synced.

Closure r1 F5: explicit OR-on-stale comment inside cleanup_old_logs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/kebab-config/src/lib.rs

@@ -443,9 +443,19 @@ pub struct LoggingCfg {
 
     /// 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,
+
+    /// v0.20.x r2 Enhancement 4: keep the most recent N ingest log files.
+    /// Older files (beyond this count) are deleted at ingest start.
+    /// Default 100. AC-9: #[serde(default)] ensures backward compat.
+    #[serde(default = "default_keep_recent_runs")]
+    pub keep_recent_runs: u32,
+
+    /// v0.20.x r2 Enhancement 4: delete log files older than N days.
+    /// Also applied to `pdf_ocr_events` SQLite rows. Default 30.
+    #[serde(default = "default_retention_days")]
+    pub retention_days: u32,
 }
 
 fn default_ingest_log_enabled() -> bool {
@@ -454,12 +464,20 @@ fn default_ingest_log_enabled() -> bool {
 fn default_ingest_log_dir() -> PathBuf {
     PathBuf::from("{state_dir}/logs")
 }
+fn default_keep_recent_runs() -> u32 {
+    100
+}
+fn default_retention_days() -> u32 {
+    30
+}
 
 impl Default for LoggingCfg {
     fn default() -> Self {
         Self {
             ingest_log_enabled: default_ingest_log_enabled(),
             ingest_log_dir: default_ingest_log_dir(),
+            keep_recent_runs: default_keep_recent_runs(),
+            retention_days: default_retention_days(),
         }
     }
 }

crates/kebab-config/tests/logging_roundtrip.rs

@@ -42,3 +42,28 @@ fn pre_v020_config_without_logging_section_gets_defaults() {
     assert!(w.logging.ingest_log_enabled);
     assert_eq!(w.logging.ingest_log_dir, PathBuf::from("{state_dir}/logs"));
 }
+
+// Test 4 (AC-9 v0.20.x r2): old config with only ingest_log_enabled + ingest_log_dir
+// parses without error and produces correct defaults for keep_recent_runs + retention_days.
+#[test]
+fn old_logging_config_parses_with_defaults() {
+    let toml = r#"
+[logging]
+ingest_log_enabled = true
+ingest_log_dir = "{state_dir}/logs"
+"#;
+    let w: LoggingWrapper = toml::from_str(toml).expect("old logging config must parse");
+    assert!(w.logging.ingest_log_enabled);
+    assert_eq!(
+        w.logging.ingest_log_dir,
+        PathBuf::from("{state_dir}/logs")
+    );
+    assert_eq!(
+        w.logging.keep_recent_runs, 100,
+        "keep_recent_runs must default to 100"
+    );
+    assert_eq!(
+        w.logging.retention_days, 30,
+        "retention_days must default to 30"
+    );
+}