A misconfigured/slow external memory provider could hold the agent in
the 'running' state for minutes after the final response was delivered.
MemoryManager.sync_all / queue_prefetch_all looped provider.sync_turn /
queue_prefetch INLINE on the turn-completion path; a provider making a
blocking network/daemon call (a broken Hindsight daemon was observed
blocking ~298s before failing) blocked run_conversation from returning.
Because every interface (CLI, TUI, gateway) marks the agent 'running'
until run_conversation returns, the agent stayed busy for the full block
and any follow-up message triggered an aggressive interrupt that dropped
the message.
Dispatch provider sync/prefetch to a lazily-created single-worker
background executor. sync_all / queue_prefetch_all return immediately;
work completes (or fails, logged) in the background. A single worker
serializes writes so turn N lands before turn N+1. flush_pending()
provides a barrier for session boundaries and deterministic tests.
shutdown_all() drains the executor with a bounded timeout so a wedged
provider can never hang teardown.
Builtin-only / no-provider sessions spawn no executor (zero new threads
in the common case).
