Navi previously had no way to write to memory mid-conversation — she
could only search and forget. Facts were extracted automatically after
sessions went idle for 30+ min, so important context shared by the user
could be lost or delayed.

- New MemorySaveTool (navi/tools/memory_save.py): upsert a fact by
  category/key/value; overwrites existing key so no separate forget needed
- Registered as builtin alongside memory_search/memory_forget
- Added to all three profiles (secretary, server_admin, smart_home)
- persona.txt: explicit "call memory_save immediately when..." guidance
  so Navi saves stable facts as they arrive, not only post-session

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>