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

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

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+# Leaderboards — `client.leaderboards`
+
+`LeaderboardsService` (source: `src/leaderboards/LeaderboardsService.ts`).
+Plain call-and-response service, NOT observable — refetch explicitly.
+
+## Surface
+
+```ts
+const board = client.leaderboards.findBySlug('weekly-kills'); // cached handle
+await board.submit(score);
+const entries = await board.list(limit?);
+board.getEntries();
+```
+
+## `LeaderboardHandle`
+
+```ts
+class LeaderboardHandle {
+  readonly slug: string;
+
+  getEntries(): readonly RankEntry[];             // last fetched list, [] initially
+  submit(score: number): Promise<void>;
+  list(limit = 100): Promise<readonly RankEntry[]>; // fetches and caches
+}
+```
+
+- `findBySlug(slug)` caches handles per slug — repeated calls return the same
+  instance (and its cached entries).
+- `list(limit)`: `limit <= 0` sends no limit to the server; default is 100.
+- `submit` does not update the cached entries; call `list()` afterwards to see
+  the effect.
+
+## Types
+
+```ts
+interface RankEntry {
+  playerId?: string;
+  playerName?: string;
+  rank?: number;
+  score?: number;
+}
+```
+
+## Notes
+
+- No player-around-me or metadata endpoints are exposed by this SDK — submit
+  and top-N list only.
+- Scenario `leaderboard` nodes surface through
+  `client.effects.onLeaderboard` (`end()`, `rewardClaimed()`) — see
+  `reference/scenarios.md`.