Adds optional `tryHandleOutOfBand` on AgentSession so providers can
handle slash commands as side effects without allocating a turn or
tripping the activeForegroundTurnId gate. Codex provider implements it
for /goal (set/pause/resume/clear), version-gated to codex >= 0.128.0
with `--enable goals`. Mid-turn /goal pause now works alongside a live
turn against the same thread.
E2E test against real codex 0.128.0 covers set + mid-turn pause + clear.
