The god-file Phase 4 refactor (094aa85c37) moved agent construction into
CLIAgentSetupMixin, which set the atexit shutdown reference with a bare
`global _active_agent_ref`. After extraction that global binds the *mixin
module's* namespace, not cli.py's. cli._run_cleanup reads
cli._active_agent_ref to decide whether to fire the memory provider's
on_session_end hook — and it stayed None for the whole session, so the
`if _active_agent_ref:` branch was dead and on_session_end never ran on
/exit. Custom memory providers silently lost end-of-session extraction.
Fix: publish the reference onto the cli module explicitly
(`import cli as _cli; _cli._active_agent_ref = self.agent`), using the
deferred-import pattern already established in the mixin.
Regression test asserts cli._active_agent_ref is populated by the mixin's
publish line and guards against a relapse to the bare `global` form. The
existing shutdown tests passed only because they hand-assigned the ref,
which is exactly what masked this.
