chore(core-realtime): scaffold package

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-realtime/AGENTS.md

@@ -0,0 +1,19 @@
+# @repo/core-realtime
+
+Vendor-isolated realtime abstractions over Socket.IO. Feature packages depend only on the interfaces; only this package imports `socket.io`.
+
+See `docs/superpowers/specs/2026-05-08-realtime-design.md` for the full design and `docs/decisions/adr-016-realtime-layer.md` for the rationale (added during implementation).
+
+## Public exports
+
+- `IRealtimeBroadcaster` — server → client broadcasts
+- `IRealtimeServer` — lifecycle, used at app boot only
+- `IRealtimeAuthenticator` — connect-time identity attachment (cookie / header → user)
+- `IRealtimeHandlerRegistry` + `RealtimeHandlerRegistry` — inbound handler registration
+- `defineRealtimeChannel`, `RealtimeChannelDescriptor`, `ChannelScope`
+- `InMemoryRealtimeBroadcaster` (test/dev), `SocketIORealtimeBroadcaster`, `SocketIORealtimeServer` (production)
+- `CORE_REALTIME_SYMBOLS`
+
+## Boundary
+
+Tagged `core`. The only place in the repo where `import "socket.io"` is allowed is `src/socket-io-*.ts` here, plus `apps/*/server.ts`. Enforced by the ESLint rule `core-eslint/no-direct-socket-io`.