feat: Add memory persistence functions (save/load-memory-to-disk)

- Add save-memory-to-disk and load-memory-from-disk to memory.lisp
- Integrate auto-save into heartbeat (every N intervals)
- Load memory on daemon startup, save on graceful shutdown/SIGINT
- Add exports to package.lisp

NOTE: Hash table serialization requires object walker for complex structures.
      Current implementation fails on load due to unreadable objects.

library/package.lisp

@@ -40,6 +40,8 @@
    #:org-object-hash
    #:snapshot-memory
    #:rollback-memory
+   #:save-memory-to-disk
+   #:load-memory-from-disk
    
    ;; --- Context API (Peripheral Vision) ---
    #:context-query-store