Docs: skills/README/CHANGELOG for effects client rewrite

- scenarios.md rewritten for server-side execution + effects API
- realtime.md deleted; realtime/planstore references purged from docs
- CHANGELOG Unreleased: major bump + migration table

CHANGELOG.md

@@ -1,5 +1,51 @@
 # Changelog
 
+## Unreleased
+
+Breaking change — major bump. The local scenario engine is replaced by a
+server-driven effects client; the realtime websocket client is removed.
+
+### Removed
+
+- Local scenario runtime: `PlanRun`, `ExecutionPlan` walking, node sessions
+  (`NotificationSession`, `StoreOfferSession`, `LeaderboardSession`,
+  `WaitSession`, `QuestSession`, `BattlePassSession`,
+  `BattlePassLevelSession`, `ConfigChangedSession`), `RespondAsync` /
+  `Respond`, `RestoreAsync`, `Clear`, `ActiveRuns`, `IsRunning`,
+  `CurrentNodeId`, `OnConfigChanged`.
+- `RudderClientOptions.PlanStateStore` / `IPlanStateStore` and
+  `IPlanScheduler` — no local plan persistence or delayed plan work.
+- `RealtimeService` (`client.Realtime`), `RudderClientOptions.RealtimeUrl` /
+  `RealtimeTransportFactory`, `IRealtimeTransport` /
+  `IRealtimeTransportFactory`.
+
+### Changed (breaking)
+
+- `client.Scenario` is a trigger only: `TriggerAsync(eventName)` returns
+  `Task` (not started plans). Returned pending effects are ingested into
+  `client.Effects`.
+- Subscribe to `client.Effects.On*` instead of `client.Scenario.On*`.
+  Sessions are now effect objects (`NotificationEffect`, `StoreOfferEffect`,
+  …). Complete them with the methods on the effect (`DoneAsync`,
+  `PurchaseAsync`/`DeclineAsync`, `EndAsync`/`ClaimAsync`, …).
+- `client.Update(deltaTime)` is still required every frame — it now pumps
+  the effects client (30s `GET /sdk/v1/scenarios/pending` heartbeat +
+  wait-deadline checks), not a local DAG or a websocket.
+- After login (or when constructed with a stored access token), the next
+  `Update` fetches pending effects. There is no `loginEvent` option; trigger
+  login-gated scenarios yourself.
+
+### Migration
+
+| Before | After |
+|---|---|
+| `client.Scenario.OnNotification` (and other `On*`) | `client.Effects.OnNotification` (same event names) |
+| `NotificationSession` / `StoreOfferSession` / … | `NotificationEffect` / `StoreOfferEffect` / … |
+| `session.CompleteAsync()` / `RespondAsync("output")` | `effect.DoneAsync()` (and the matching method on each effect) |
+| `RudderClientOptions.PlanStateStore` + `RestoreAsync` | gone — server owns run state; pending fetch after login + heartbeat |
+| `client.Realtime` | gone |
+| `client.Update(deltaTime)` | still required (effects pump) |
+
 ## 0.4.0
 
 - Generated DTOs live under `Models/` (`RudderSdk.Core.Models`). Wire fields