feat: add anthropic oauth authentication to claude code sdk (#2)

[acebytes]

• feat: Add programmatic authentication support via OAuth flow

• Implement OAuth PKCE authentication flow based on opencode implementation
• Add ClaudeAuth and AnthropicAuth classes for programmatic auth
• Store credentials securely in ~/.claude/credentials.json with 600 permissions
• Automatic token refresh when expired
• Complete auth example with interactive flow
• Export auth functions: startAuth, isAuthenticated, getAccessToken, logout
• Add comprehensive test coverage for auth functionality
• Update README with detailed authentication documentation

Resolves #10

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• feat: implement Anthropic auth flow with OpenAuth.js

• refactor: move auth flow to auth directory and simplify implementation

• refactor: extract auth storage interface and implement file-based storage

• feat: store OAuth credentials in file storage after token exchange

• refactor: simplify auth flow by separating token refresh from storage

• renamed AuthAnthropic.access() to refresh() for clarity
• removed storage dependency from refresh method
• moved token storage logic to auth-example.ts
• refresh now only handles token refresh, not storage

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• feat: add in-memory authentication example

• new example that doesn't use filesystem storage
• outputs credentials directly to stdout
• demonstrates token refresh functionality
• useful for temporary auth or CI/CD environments

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• refactor: update import paths to use published npm package instead of local source

• chore: remove unused dependencies (crypto, node-fetch)

removed packages that are not actually used in the implementation:

• crypto: using node's built-in crypto module
• node-fetch: using native fetch api

🤖 generated with claude code

• refactor: remove AuthFileStorage, keep validation helpers

• removed AuthFileStorage class for manual storage handling
• kept zod validation schemas as exported helpers
• updated auth-example.ts to use manual file operations
• exported OAuthCredentials, validateCredentials, isValidCredentials

🤖 generated with claude code

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