Fix ox-hugo export with internal ID links and Quartz config

- Add org-id resolution for [[id:...]] links by building ID locations
  database once before parallel export
- Handle broken file links gracefully with org-export-with-broken-links
- Fix race condition in parallel exports by pre-building ID cache
- Fix Quartz config not being applied: cp was preserving nix store hash
  prefix in filename instead of using explicit destination filename
- Continue pipeline even when some exports fail, reporting failures
- Improve error handling and logging throughout export pipeline

lib/org_garden/watcher.ex

@@ -182,8 +182,8 @@ defmodule OrgGarden.Watcher do
         duration = System.monotonic_time(:millisecond) - start_time
         Logger.info("  done in #{duration}ms")
 
-      {:error, reason} ->
-        Logger.error("Watcher: export failed for #{orgfile}: #{inspect(reason)}")
+      {:error, {:emacs_exit, code}} ->
+        Logger.error("Watcher: export failed for #{Path.basename(orgfile)} (exit code #{code})")
     end
   end