Adds remote_sync_queue.go with persistent queue + 1s→30s exponential
backoff drainer, max-attempt drop with audit record, ctx.Done() clean
shutdown. sync.go rewired to enqueue dispatches via the queue rather
than the legacy minute-ticker schedule.
CORE_AGENT_SYNC_MAX_ATTEMPTS / CORE_SYNC_MAX_ATTEMPTS env vars override
the default 100-attempt cap.
Targeted tests cover happy-path drain, retry/backoff with stub clock,
cancellation, max-attempt exhaustion, and file-backed restart persistence.
go test was blocked by go.work workspace dep resolution in sandbox; CI
catches the real run.
Co-authored-by: Codex <noreply@openai.com>
Closes tasks.lthn.sh/view.php?id=231
