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/main.rs

@@ -14,6 +14,7 @@
 // interference detection, schema assimilation, reconsolidation.
 
 mod store;
+mod util;
 mod llm;
 mod digest;
 mod audit;
@@ -739,8 +740,7 @@ fn cmd_apply_agent(args: &[String]) -> Result<(), String> {
 
         // Move processed file to avoid re-processing
         if !process_all {
-            let done_dir = results_dir.join("done");
-            std::fs::create_dir_all(&done_dir).ok();
+            let done_dir = util::memory_subdir("agent-results/done")?;
             let dest = done_dir.join(path.file_name().unwrap());
             std::fs::rename(&path, &dest).ok();
         }
@@ -793,7 +793,7 @@ fn cmd_digest(args: &[String]) -> Result<(), String> {
 fn cmd_digest_links(args: &[String]) -> Result<(), String> {
     let do_apply = args.iter().any(|a| a == "--apply");
 
-    let links = digest::parse_all_digest_links();
+    let links = digest::parse_all_digest_links()?;
     println!("Found {} unique links from digest files", links.len());
 
     if !do_apply {