feat: implement ADR-007 subscription transport — PendingRequestMap.subscribe(), CallHandler dispatch, SSE AsyncGenerator handlers

Add remote subscription support so spokes can consume streaming operations
over pubsub transports (WebSocket, Redis). Extract checkAccess to access.ts
to break circular dep between call.ts and subscribe.ts.

tasks/implement-adr-007-subscription-transport.md

@@ -6,6 +6,8 @@
 
 **Architecture docs**: [ADR-007](../docs/architecture/decisions/007-subscription-transport.md), [call-protocol.md](../docs/architecture/call-protocol.md), [adapters.md](../docs/architecture/adapters.md)
 
+**Status**: ✅ Completed (2026-05-16)
+
 ## Scope
 
 Three changes, all in source. No new modules needed.