/**
 * Merge config environment variables onto process.env for subprocess spawning.
 *
 * On Windows, environment variable names are case-insensitive at the OS level,
 * but a plain JS object treats "PATH" and "Path" as separate keys. When both
 * exist in the spawned env, the behavior is unpredictable — the OS may resolve
 * the duplicate in an unintended order, silently dropping the caller's override.
 *
 * This function deduplicates keys case-insensitively on Windows so that
 * `override` values reliably replace matching `base` keys. On POSIX systems,
 * keys are case-sensitive and a simple spread is sufficient.
 *
 * Modeled after VS Code's `mergeEnvironments()` in
 * `src/vs/workbench/contrib/terminal/common/terminalEnvironment.ts`.
 */
export declare function mergeEnv(base: NodeJS.ProcessEnv, override?: Record<string, string>): Record<string, string>;
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