feat(core): add mergeAdvanceAutoSync project setting

Schema for what the merger should do in other worktrees still checked out
on the integration branch when it advances the branch ref. Modes:
  off          — legacy (user pulls manually)
  ff-only      — fast-forward only when other worktree is clean
  stash-and-ff — Smart Pull pipeline (default)

Threads through DEFAULT_PROJECT_SETTINGS, PROJECT_SETTINGS_KEYS (auto via
Object.keys), the docs settings table, and parity + persistence tests.
Merger consumption lands in the follow-up engine change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/core/src/__tests__/settings-parity.test.ts

@@ -157,6 +157,12 @@ describe("settings key parity", () => {
     expect(isGlobalSettingsKey("directMergeCommitStrategy")).toBe(false);
   });
 
+  it("defaults mergeAdvanceAutoSync to stash-and-ff and keeps it project-scoped", () => {
+    expect(DEFAULT_PROJECT_SETTINGS.mergeAdvanceAutoSync).toBe("stash-and-ff");
+    expect(isProjectSettingsKey("mergeAdvanceAutoSync")).toBe(true);
+    expect(isGlobalSettingsKey("mergeAdvanceAutoSync")).toBe(false);
+  });
+
   it("keeps integrationBranch project-scoped", () => {
     expect(DEFAULT_PROJECT_SETTINGS.integrationBranch).toBeUndefined();
     expect(isProjectSettingsKey("integrationBranch")).toBe(true);

packages/core/src/__tests__/store-settings.test.ts

@@ -111,6 +111,15 @@ describe("TaskStore", () => {
       const settings = await harness.store().getSettings();
       expect(settings.directMergeCommitStrategy).toBe("always-rebase");
     });
+
+    it("defaults mergeAdvanceAutoSync to stash-and-ff and persists updates", async () => {
+      const defaults = await harness.store().getSettings();
+      expect(defaults.mergeAdvanceAutoSync).toBe("stash-and-ff");
+
+      await harness.store().updateSettings({ mergeAdvanceAutoSync: "off" });
+      const settings = await harness.store().getSettings();
+      expect(settings.mergeAdvanceAutoSync).toBe("off");
+    });
   });
 
   // ── Planning/Validator Model Settings ────────────────────────────

packages/core/src/settings-schema.ts

@@ -197,6 +197,7 @@ export const DEFAULT_PROJECT_SETTINGS = {
   mergeStrategy: "direct",
   directMergeCommitStrategy: "always-squash",
   mergeIntegrationWorktree: "reuse-task-worktree",
+  mergeAdvanceAutoSync: "stash-and-ff",
   integrationBranch: undefined,
   requirePrApproval: false,
   pushAfterMerge: false,

packages/core/src/types.ts

@@ -213,6 +213,12 @@ export function normalizeMergeIntegrationWorktreeMode(
 
 export const DIRECT_MERGE_COMMIT_STRATEGIES = ["auto", "always-squash", "always-rebase"] as const;
 export type DirectMergeCommitStrategy = (typeof DIRECT_MERGE_COMMIT_STRATEGIES)[number];
+
+export const MERGE_ADVANCE_AUTO_SYNC_MODES = ["off", "ff-only", "stash-and-ff"] as const;
+export type MergeAdvanceAutoSyncMode = (typeof MERGE_ADVANCE_AUTO_SYNC_MODES)[number];
+export function normalizeMergeAdvanceAutoSyncMode(value: unknown): MergeAdvanceAutoSyncMode {
+  return value === "off" || value === "ff-only" || value === "stash-and-ff" ? value : "stash-and-ff";
+}
 /** How merge conflicts are resolved when the AI agent can't (or shouldn't) decide.
  *
  *  Both `smart-*` strategies share the same cascade: pre-merge fetch +
@@ -2743,6 +2749,21 @@ export interface ProjectSettings {
    *  - "cwd-main": legacy alias for "cwd-integration-branch" (normalized at read time)
    *  Auto-merge only; manual/direct merge entrypoints outside auto-merge are unchanged. */
   mergeIntegrationWorktree?: MergeIntegrationWorktreeMode;
+  /** After the merger advances the integration branch ref, what to do in *other*
+   *  worktrees that have the same branch checked out (typically the user's
+   *  project-root checkout that sat at the previous tip).
+   *  - "off": do nothing; the user must `git pull` (or click the Merge Advance
+   *    Notice banner's Pull button) to bring their checkout forward.
+   *  - "ff-only": fast-forward the other worktree only when its index and
+   *    working tree are clean. Dirty worktrees are left alone and the banner
+   *    still surfaces for manual handling.
+   *  - "stash-and-ff" (default): run the Smart Pull pipeline
+   *    (stash → fast-forward → pop) so local edits survive across the
+   *    auto-sync. Pop conflicts surface as `merge:auto-sync` audit events with
+   *    `outcome: "stash-pop-conflict"` and are forwarded to the dashboard's
+   *    existing stash-conflict modal.
+   *  Only applies to direct merges (`mergeStrategy === "direct"`). */
+  mergeAdvanceAutoSync?: MergeAdvanceAutoSyncMode;
   /** Explicit integration branch name (e.g. `main`, `master`, `trunk`, `develop`).
    *  Resolution order: `integrationBranch` → `baseBranch` → `origin/HEAD` → `main`.
    *  This value is used as the `projectDefaultBranch` input to `resolveTaskMergeTarget`. */