/**
 * Write or update an environment variable in a .env file
 *
 * Uses atomic write pattern (temp file + rename) to prevent corruption
 * if process crashes during write.
 *
 * ⚠️ **SECURITY WARNING**: This function stores values in PLAINTEXT.
 * Never use for production secrets. Use environment variables,
 * secret managers (AWS Secrets Manager, Vault), or encrypted files.
 *
 * **Thread Safety**: Not safe for concurrent writes to the same file.
 * Multiple processes writing simultaneously may cause race conditions.
 *
 * **Format Assumptions**: Assumes KEY=VALUE format without quotes or inline comments.
 *
 * @param filePath - Path to the .env file
 * @param key - Environment variable key (uppercase recommended)
 * @param value - Environment variable value (stored as-is)
 *
 * @example
 * ```typescript
 * // Create new variable
 * await writeEnvVar('.env', 'DATABASE_URL', 'postgresql://localhost/mydb');
 *
 * // Update existing variable
 * await writeEnvVar('.env', 'DATABASE_URL', 'postgresql://localhost/newdb');
 *
 * // Result is idempotent
 * await writeEnvVar('.env', 'API_KEY', 'abc123');
 * await writeEnvVar('.env', 'API_KEY', 'abc123'); // No change
 * ```
 */
export declare function writeEnvVar(filePath: string, key: string, value: string): Promise<void>;