Server-side scenario execution: thin effects client replaces local engine

- engine/ (DagWalker, sessions, IndexedDbPlanStore) deleted; server owns the graph
- trigger/callback/counter carry PendingEffect; GET /sdk/v1/scenarios/pending polled
  (30s jittered heartbeat paused on hidden tab + wait-deadline timers)
- client.effects public API unchanged (done/buy/dismiss/end/claim/battlepass/quest)
- planStateStore removed; loginEvent kept; reconcile() makes server the source of truth
- generated models regenerated; skills/README/CHANGELOG updated

CHANGELOG.md

@@ -1,5 +1,19 @@
 # Changelog
 
+## Unreleased
+
+- Scenario execution moved server-side. The SDK no longer walks a local DAG
+  or persists plan state to IndexedDB.
+- Removed `RudderClientOptions.runtime.planStateStore` and the
+  `PlanStateStore` type.
+- `GET /sdk/v1/scenarios/pending` is polled on login, on a ~30s heartbeat,
+  and at each effect `waitDeadline`. Completions POST
+  `/sdk/v1/scenarios/callback`; the response may carry the next
+  `PendingEffect`.
+- Scenario `remote_config_override` nodes no longer emit `onConfigChanged`
+  (the server applies the override). The `ConfigChangedEffect` type and
+  `onConfigChanged` subscription remain.
+
 ## 0.6.0
 
 - `LeaderboardSession.claim()` / effect `claim()` completes the leaderboard