The three big review-prompt strings (_MEMORY_REVIEW_PROMPT,
_SKILL_REVIEW_PROMPT, _COMBINED_REVIEW_PROMPT — 183 lines combined) move
out of the AIAgent class body and into agent/background_review.py where
they're consumed.
AIAgent re-exposes them as class attributes via 'from ... import' inside
the class body — Python binds those names into the class namespace so
existing AIAgent._MEMORY_REVIEW_PROMPT references keep working.
spawn_background_review_thread also falls back to the module-level
constants if an agent doesn't have the attribute (preserves the test
pattern of mocking these on the agent).
tests/run_agent/ + tests/agent/: 4313 passed (same pre-existing
test_auxiliary_client failure).
run_agent.py: 9986 -> 9800 lines (-186).
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