replace libc date math with chrono, extract memory_subdir helper

- date_to_epoch, iso_week_info, weeks_in_month: replaced unsafe libc
  (mktime, strftime, localtime_r) with chrono NaiveDate and IsoWeek
- epoch_to_local: replaced unsafe libc localtime_r with chrono Local
- New util.rs with memory_subdir() helper: ensures subdir exists and
  propagates errors instead of silently ignoring them
- Removed three duplicate agent_results_dir() definitions across
  digest.rs, consolidate.rs, enrich.rs
- load_digest_files, parse_all_digest_links, find_consolidation_reports
  now return Result to properly propagate directory creation errors

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/enrich.rs

@@ -16,11 +16,7 @@ use std::collections::hash_map::DefaultHasher;
 use std::fs;
 use std::hash::{Hash, Hasher};
 
-fn agent_results_dir() -> std::path::PathBuf {
-    let dir = store::memory_dir().join("agent-results");
-    fs::create_dir_all(&dir).ok();
-    dir
-}
+use crate::util::memory_subdir;
 
 /// Extract user/assistant messages with line numbers from a JSONL transcript.
 /// (line_number, role, text, timestamp)
@@ -194,7 +190,7 @@ pub fn journal_enrich(
     // Save result to agent-results
     let timestamp = store::format_datetime(store::now_epoch())
         .replace([':', '-'], "");
-    let result_file = agent_results_dir()
+    let result_file = memory_subdir("agent-results")?
         .join(format!("{}.json", timestamp));
     let output = serde_json::json!({
         "timestamp": timestamp,