/**
 * FF-only git merge utility for the Tier-3 merge ritual.
 *
 * Enforces the linear-history contract: merges MUST be fast-forward only.
 * If the experiment branch has diverged from the target (i.e. cannot be
 * fast-forwarded), the merge is aborted immediately. The codebase NEVER
 * auto-rebases — the caller must resolve divergence manually and retry.
 *
 * Kill-switch checks are performed at `pre-merge` and `post-merge` step
 * boundaries via {@link checkKillSwitch}. A kill between the merge commit
 * landing and the post-merge check cannot be auto-reverted by this utility;
 * the caller is responsible for manual cleanup in that case.
 *
 * Algorithm:
 *   1. `checkKillSwitch('pre-merge')` — abort before touching git
 *   2. `git -C <targetBranchCwd> merge --ff-only <experimentRef>`
 *   3. Non-zero exit → `git merge --abort` (safety net), return
 *      `{ merged: false, reason: 'ff-failed-abort' }`
 *   4. `checkKillSwitch('post-merge')` — warn but cannot auto-revert
 *   5. Return `{ merged: true, headSha }`
 *
 * Subprocess safety:
 * - All git arguments are passed as separate array elements to
 *   `child_process.spawn`. No shell interpolation of user-supplied strings.
 * - `shell: false` (Node default) ensures paths with spaces or special
 *   characters cannot escape into shell metacharacter injection.
 *
 * @see ADR-054 — Sentient Loop Tier-3 (sandbox auto-merge)
 * @task T1028
 */
/** Result returned by {@link gitFfMerge}. */
export interface MergeResult {
    /** Whether the fast-forward merge succeeded and was committed. */
    merged: boolean;
    /**
     * HEAD SHA of the target branch after the operation.
     *
     * - When `merged: true` — the new HEAD after the ff-merge.
     * - When `merged: false` — the unchanged HEAD (merge was aborted).
     * - Empty string on unexpected git failure.
     */
    headSha: string;
    /**
     * Reason for a non-merged outcome.
     *
     * - `'ff-failed-abort'` — `git merge --ff-only` exited non-zero; histories
     *   have diverged. The caller MUST resolve divergence manually.
     * - `'kill-switch-activated'` — kill switch fired at `pre-merge`; merge was
     *   never attempted.
     * - `'verify-failed'` — git subprocess could not be spawned or an unexpected
     *   error occurred during the merge operation.
     */
    reason?: 'ff-failed-abort' | 'kill-switch-activated' | 'verify-failed';
}
/** Options for {@link gitFfMerge}. */
export interface GitFfMergeOptions {
    /**
     * Absolute path to the experiment worktree (the directory that contains the
     * experiment branch's work tree). Used to derive the experiment ref.
     */
    experimentWorktree: string;
    /**
     * Name of the target branch (e.g. `'main'`). The merge is performed inside
     * the working directory of the target branch — callers must ensure `cwd`
     * resolves to a worktree that has `targetBranch` checked out.
     *
     * The merge command run is:
     * ```
     * git -C <targetBranchCwd> merge --ff-only <experimentRef>
     * ```
     * where `<targetBranchCwd>` is resolved from `experimentWorktree` via
     * the `cwd` option below, or defaults to `process.cwd()`.
     */
    targetBranch: string;
    /**
     * Working directory for git commands. Should be a worktree that has
     * `targetBranch` checked out.
     *
     * Defaults to `process.cwd()`.
     */
    cwd?: string;
    /**
     * Override for the git binary path. Defaults to `'git'` (from PATH).
     * Useful for testing with a fake git script.
     */
    gitBin?: string;
}
/**
 * Perform a fast-forward-only git merge of an experiment worktree into the
 * target branch.
 *
 * The merge is the final gating action of the Tier-3 ritual. This function
 * enforces two invariants:
 *
 * 1. **Linear history only** — if `git merge --ff-only` exits non-zero, the
 *    merge is immediately aborted (`git merge --abort`) and
 *    `{ merged: false, reason: 'ff-failed-abort' }` is returned. The caller
 *    MUST resolve the divergence manually before retrying.
 *
 * 2. **Kill-switch gates** — the kill switch is checked at `pre-merge` before
 *    any git command is issued, and at `post-merge` after a successful merge.
 *    A `post-merge` kill-switch activation cannot be auto-reverted; the caller
 *    receives `{ merged: true, headSha }` and must handle cleanup.
 *
 * @param options - See {@link GitFfMergeOptions}.
 * @returns {@link MergeResult} describing the outcome.
 */
export declare function gitFfMerge(options: GitFfMergeOptions): Promise<MergeResult>;
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