feat: Add AuthClient and LeaderboardClient to TypeScript SDK (Closes #863)

[sepulchralvoid666]

What this does

Completes the SolFoundry TypeScript SDK by adding two missing API clients and full test coverage.

New: AuthClient (sdk/src/auth.ts)

Full GitHub OAuth 2.0 authentication lifecycle:

• getGitHubAuthorizeUrl() — Fetch GitHub OAuth authorize URL for redirecting users
• exchangeGitHubCode(code, state?) — Exchange OAuth callback code for JWT tokens + user profile
• refreshTokens(refreshToken) — Refresh expired access tokens without re-auth
• getMe() — Fetch current authenticated user profile

All methods include full JSDoc with @throws annotations and @example blocks.

New: LeaderboardClient (sdk/src/leaderboard.ts)

Platform rankings and statistics:

• getLeaderboard(period?) — Fetch contributor leaderboard for 7d/30d/90d/all time periods
• getStats() — Platform-wide statistics with field name normalization (handles both open_bounties and active_bounties field shapes)

Updated: SolFoundry Facade

The main SolFoundry class now exposes:

• client.auth — AuthClient instance
• client.leaderboard — LeaderboardClient instance

Updated JSDoc examples show both new clients in action.

New: Barrel Exports

All new types and classes exported from sdk/src/index.ts:

• AuthClient, AuthTokens, GitHubAuthResponse, AuthUser, AuthClientConfig
• LeaderboardClient, TimePeriod, LeaderboardEntry, PlatformStats

New: Test Suite

• sdk/src/__tests__/auth.test.ts — 6 tests covering all AuthClient methods, error propagation, and optional params
• sdk/src/__tests__/leaderboard.test.ts — 6 tests covering leaderboard fetch, paginated response handling, stats normalization, and zero-defaults

New: Example

• sdk/examples/03-github-auth.ts — Full OAuth walkthrough: authorize URL → code exchange → token refresh → leaderboard + stats

How to test

1. Review sdk/src/auth.ts and sdk/src/leaderboard.ts for API coverage completeness
2. Review test files in sdk/src/__tests__/
3. Run cd sdk && npm install && npm test to execute all tests
4. Review examples/03-github-auth.ts for usage patterns
5. Verify the SolFoundry facade now includes auth and leaderboard properties

SDK API Coverage Summary

Client	Methods	Status
BountyClient	list, get, create, update, delete, search, autocomplete, submit	Existing
EscrowClient	fund, release, refund, getStatus, getLedger	Existing
ContributorClient	create, get, update, list, getStats	Existing
AuthClient	getGitHubAuthorizeUrl, exchangeGitHubCode, refreshTokens, getMe	New
LeaderboardClient	getLeaderboard, getStats	New
GitHubClient	listBounties, checkClaim, verifyCompletion	Existing
EventSubscriber	connect, subscribe, on, disconnect	Existing

Closes #863

Wallet: 2JHa4QQWNV7AGSGsVeX1cwjZtFcmTTomb1TrJmQthoh3