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>

.changeset/merge-advance-auto-sync-setting.md

@@ -0,0 +1,14 @@
+---
+"@fusion/core": minor
+---
+
+feat(core): add `mergeAdvanceAutoSync` project setting (`"off" | "ff-only" | "stash-and-ff"`)
+
+Adds the schema for a new project setting that controls what happens in **other** worktrees still checked out on the integration branch when the merger advances the branch ref. Previously the merger only updated `refs/heads/<branch>` and left every other checkout's index and working tree pinned at the old tip, so `git status` in the user's project-root checkout reported the new commits as inverted "staged changes to be committed."
+
+Modes (default `"stash-and-ff"`):
+- `"off"` — preserve the legacy behavior; user must `git pull` or click the Merge Advance Notice banner Pull button.
+- `"ff-only"` — auto-fast-forward only clean worktrees; dirty worktrees stay untouched and the banner still surfaces.
+- `"stash-and-ff"` — run the Smart Pull pipeline (stash → fast-forward → pop). Pop conflicts emit `merge:auto-sync` audit events with `outcome: "stash-pop-conflict"` and surface through the existing dashboard stash-conflict modal.
+
+Schema-only in this changeset; the merger hook that consumes the setting lands in the follow-up engine change.