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/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,