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

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

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+# Inventory + catalog — `client.inventory`, `client.catalog`
+
+## Inventory
+
+`InventoryDomain` (source: `src/domains/InventoryDomain.ts`) — owned items
+merged with their catalog entries. Synced under revision key `inventory`;
+also refreshed whenever the catalog changes.
+
+```ts
+client.inventory.data   // InventoryItem[] | undefined
+client.inventory.onChange(cb);
+await client.inventory.load() / .reload();
+```
+
+### `InventoryItem` (merged view, source: `src/state/inventory.ts`)
+
+```ts
+interface InventoryItem {
+  slug: string;                        // '' when the wire item has no slug
+  amount: number;                      // 0 when absent on the wire
+  name: string;                        // nameOverride || catalog name || slug
+  properties: Record<string, unknown>; // catalog properties + propertiesOverride (override wins)
+  tags: string[];                      // from the catalog entry
+}
+```
+
+The raw wire shape is `PlayerInventoryItem`
+(`slug?, amount?, nameOverride?, propertiesOverride?, updatedAt?`) — you rarely
+need it; the domain hands out the merged `InventoryItem`.
+
+## Catalog
+
+`CatalogDomain` (source: `src/domains/CatalogDomain.ts`) — the item catalog
+keyed by slug. Synced under revision key `catalog`.
+
+```ts
+client.catalog.data   // Map<string, CatalogItem> | undefined
+
+interface CatalogItem {
+  name?: string;
+  properties?: { [key: string]: unknown };
+  slug?: string;
+  tags?: string[];
+}
+```
+
+Items without a `slug` are skipped when the map is built.
+
+## Behavior notes
+
+- The catalog is warmed at login; inventory loads on first use, waits on the
+  catalog load, and merges, so `client.inventory.data` always has catalog
+  fields filled in.
+- Invalidated after store purchases and scenario callbacks.
+- Inventory has no client-side mutations — items change via purchases, quest /
+  battle pass rewards, and scenario nodes (all server-side).