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

tsup.config.ts

@@ -6,7 +6,7 @@ export default defineConfig({
   target: 'es2022',
   dts: true,
   clean: true,
-  // Code-split the lazily-imported scenario engine out of the initial chunk.
+  // Code-split the lazily-imported scenario client out of the initial chunk.
   splitting: true,
   treeshake: true,
   platform: 'browser',