/**
 * mergeGitignore computes the .gitignore content `diablo init` should write,
 * idempotently. diablo creates machine-managed runtime dirs (.diablo/,
 * .worktrees/) that must never be committed in ANY project it scaffolds, so
 * those always land in a clearly-marked managed block.
 *
 * Pure (string in, string-or-null out) so the merge rules are unit-tested
 * directly; the adapter only reads the existing file and writes the result.
 */
/**
 * Computes the .gitignore content for `diablo init`.
 *
 * @param existing the current .gitignore content, or null if none exists.
 * @param greenfield true when the repo has no commits yet — a fresh project,
 *   where diablo also seeds common ignores (node_modules/, dist/, .env) since
 *   nothing has established them. Brownfield repos keep their own conventions;
 *   diablo only contributes its runtime dirs.
 * @returns the content to write, or null when nothing needs to change (the
 *   managed block is already present).
 */
export declare function mergeGitignore(existing: string | null, greenfield: boolean): string | null;