Add agent skill (SKILL.md + per-domain reference)

skills/rudder-web-sdk/reference/player.md

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+# Player profile + wallets — `client.player`
+
+`PlayerDomain` (source: `src/domains/PlayerDomain.ts`) — the observable player
+profile: identity plus currency wallets. Synced under revision key `profile`.
+
+There is no separate wallet service in this SDK. Wallet balances are read from
+the profile and are credited/debited server-side (purchases, quest/battle pass
+rewards, scenario nodes).
+
+## Surface
+
+`client.player` is a `SyncedState<PlayerProfile>`:
+
+```ts
+client.player.data       // PlayerProfile | undefined
+client.player.status     // 'idle' | 'loading' | 'ready' | 'error'
+client.player.onChange((snapshot) => { /* fires immediately */ });
+await client.player.load();
+await client.player.reload();  // force refetch
+```
+
+## Types
+
+```ts
+interface PlayerProfile {
+  player?: Player;
+  wallets?: Wallet[];
+}
+
+interface Player {
+  createdAt?: string;
+  id?: string;
+  language?: string;
+  nickname?: string;
+  projectId?: string;
+  region?: string;
+}
+
+interface Wallet {
+  balance?: number;
+  currency?: string;  // currency code configured in the dashboard
+}
+```
+
+## Behavior notes
+
+- Warmed automatically at login (one of the four parallel warm loads).
+- Invalidated (refetched) after every successful store purchase and after
+  scenario server callbacks — subscribers see fresh balances without waiting
+  for the revision poll.
+- All fields are optional on the wire (`?`); code defensively.
+- React: `useSyncExternalStore` maps directly onto `onChange` (see the SDK
+  README "React recipe").