feat(core-realtime): scaffold realtime optional core

Generator-emitted scaffold (pnpm turbo gen core-package realtime) plus
the story-00-precedent coverage repairs (coverage provider devDep,
symbols.ts exclude + tested allowlist mirror) and three minimal tests
covering generator-emitted realtime code the template suite misses.
Squash of 31d85e0 + review-fix cf11b38.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

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`.
+
+ADR-016 (`docs/decisions/adr-016-realtime-layer.md`).
+
+## 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`.