Splits the 494-line attackers.py into five submixin files plus a
composing AttackersMixin in attackers/__init__.py:
_core.py (~95) Attacker CRUD + _deserialize_attacker
behavior.py (~110) AttackerBehavior + _deserialize_behavior
sessions.py (~50) SessionProfile read/write
smtp.py (~70) SmtpTarget per-attacker + cross-attacker views
activity.py (~190) log-derived activity (commands, leaks,
artifacts, stored mail, session log, transcripts)
IdentitiesMixin.list_observations_for_identity calls
self._deserialize_attacker; MRO resolves it onto AttackersCoreMixin
through the composed SQLModelRepository class.