fix(sync): close SQLite handles before restore

Use contextlib.closing for dump, comparison, and restore connections so Windows can unlink the live database immediately after the comparison step.

tests/test_smoke.py

@@ -264,13 +264,17 @@ def test_sync_compare_detects_direction(tmp_path):
     """The guard that stops a stale box from overwriting the other's chats.
     Both backup and restore refuse to run when this says the wrong thing, so a
     silent break here loses conversation history."""
+    import contextlib
     import sqlite3 as sq
     sync = _load_sync()
     db, dump = tmp_path / "memory.db", tmp_path / "memory.db.sql"
 
     def write(rows):
         db.unlink(missing_ok=True)
-        with sq.connect(db) as conn:
+        # closing() then the connection itself: sqlite3's own context manager
+        # commits but never closes, and Windows refuses to unlink a file that
+        # still has an open handle.
+        with contextlib.closing(sq.connect(db)) as conn, conn:
             # updated_at REAL, matching the production schema in store.py. A TEXT
             # column here hid a real TypeError for months: the comparison in
             # _extra() ran str-vs-str in the test and str-vs-float in the field.