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>

docs/SMOKE.md

@@ -149,6 +149,12 @@ skip_generated_header = true
 max_file_bytes = 262144
 max_file_lines = 5000
 extra_skip_globs = []                # 사용자 추가 skip 패턴 (gitignore syntax)
+
+[logging]
+ingest_log_enabled = true
+ingest_log_dir = "{state_dir}/logs"
+keep_recent_runs = 100               # v0.20.x r2: 최근 N 개 run log 파일 보존
+retention_days = 30                  # v0.20.x r2: N일 이상 된 log / OCR 이벤트 자동 삭제
 ```
 
 `KEBAB_*` 환경변수로 override 가능 (`KEBAB_MODELS_LLM_MODEL=gemma4:26b kebab …` 등). 자세한 키 목록은 `crates/kebab-config/src/lib.rs` 의 `apply_env` 매치 암. `KEBAB_READONLY=1` — write-path 비활성화 (CI 안전망). `KEBAB_PROGRESS=plain` — non-TTY 환경에서 진행 상황을 plain 한 줄씩 stderr 출력 (spinner 대신).