Replace three competing event paths (foreground stream, live event pump,
JSONL history poller) with a single push-based subscribe() + startTurn()
contract. This fixes duplicate user messages and stuck running state caused
by timing-based routing between concurrent event sources.
Key changes:
- AgentSession interface: remove stream(), add subscribe() and startTurn()
- All providers (Claude, Codex, OpenCode): single subscribers Set with
notifySubscribers() for push-based event delivery and turnId stamping
- Agent manager: identity-based turn ownership via activeForegroundTurnId
replacing pendingRun async generator
- Delete: dual queues, routeSdkMessageFromPump, startLiveHistoryPolling,
snapHistoryOffsetToEnd, liveEventBacklog, Pushable
- Fix Codex provider not clearing activeForegroundTurnId on turn completion
- Add real-provider integration tests for event stream invariants
