feat(github): add in-memory token cache for keychain reads and writes

[mohammedhammoud]

Changes

• Added OnceLock<Mutex<Option<String>>> static token cache
• Added cache_token() to write tokens into in-memory cache
• Added cached_token() to read tokens from in-memory cache
• Updated GitHubCredentials::write_token to store in cache after keychain write
• Updated read_token to check cache before falling back to keychain
• Updated keychain write path to persist to in-memory cache

Changes