A steer queued on a session with an empty transcript was undeliverable:
Agent.continue() threw "No messages to continue from" before dequeuing any
steering message, so the RPC idle-drain re-armed continue() on every microtask
(gated only on hasQueuedMessages(), which never cleared) — an unbounded
allocation loop that OOM-killed the process.
continue() now consumes a queued steer/follow-up as the opening turn when the
transcript is empty, mirroring the assistant-tail branch. The queue drains, the
re-arm's hasQueuedMessages() gate goes false, and the loop terminates.
Fixes#6344
roboomp