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

src/generated/battlepass.ts

@@ -1,6 +1,6 @@
 // Code generated by apigen. DO NOT EDIT.
 
-import type { ExecutionPlan, Reward } from './common.js';
+import type { Reward } from './common.js';
 
 export interface AddBattlePassXpRequest {
   "amount"?: number;
@@ -14,7 +14,6 @@ export interface AddBattlePassXpResponse {
   "level"?: number;
   "leveledUp"?: boolean;
   "maxLevel"?: boolean;
-  "plan"?: ExecutionPlan;
   "xp"?: number;
 }
 
@@ -59,7 +58,6 @@ export interface PurchaseBattlePassPremiumRequest {
 
 export interface PurchaseBattlePassPremiumResponse {
   "error"?: string;
-  "plan"?: ExecutionPlan;
   "success"?: boolean;
 }