Implements #6043: Add write guards for multi-agent crews to prevent
cross-agent memory poisoning.
- Add memory_guard field to Crew (Optional[Callable[[str], bool]])
- Integrate guard check into Memory base class save()
- Add guard check to LongTermMemory.save() (bypasses super)
- Propagate guard from Crew to all memory instances on creation
- Log warnings when writes are blocked
- Add 19 tests covering all memory types and Crew integration
Co-Authored-By: João <joao@crewai.com>
