Move the background-review subsystem (the self-improvement loop — see the
README) out of run_agent.py into a dedicated module.
* summarize_background_review_actions — was the @staticmethod that builds
the user-facing action summary
* spawn_background_review_thread — builds the thread target + prompt;
the actual review loop body (forked AIAgent, runtime inheritance,
tool whitelist, suppression, teardown) lives in _run_review_in_thread
* build_memory_write_metadata — provenance for external memory mirrors
AIAgent keeps thin wrappers for backward compatibility AND because tests
patch run_agent.threading.Thread to assert lifecycle behavior — the
threading.Thread construction stays in AIAgent._spawn_background_review,
the inner work moves out.
tests/run_agent/ + tests/agent/: 4313 passed, 1 pre-existing failure
(test_auxiliary_client.py::test_custom_endpoint... — confirmed failing
on main before this change). 3 skipped.
run_agent.py: 15272 -> 14972 lines (-300).
