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/util.rs

@@ -0,0 +1,14 @@
+// Shared utilities
+
+use crate::store;
+
+use std::fs;
+use std::path::PathBuf;
+
+/// Ensure a subdirectory of the memory dir exists and return its path.
+pub(crate) fn memory_subdir(name: &str) -> Result<PathBuf, String> {
+    let dir = store::memory_dir().join(name);
+    fs::create_dir_all(&dir)
+        .map_err(|e| format!("create {}: {}", dir.display(), e))?;
+    Ok(dir)
+}