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>

Cargo.toml

@@ -10,6 +10,7 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 bincode = "1"
 regex = "1"
+chrono = "0.4"
 libc = "0.2"
 faer = "0.24.0"
 rkyv = { version = "0.7", features = ["validation", "std"] }