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.

bin/sync.py

@@ -18,6 +18,7 @@ bin/db-compare.sh could never work there.
 A fresh machine clones first (git clone <repo> nexus-core), then runs this.
 """
 import argparse
+import contextlib
 import os
 import re
 import shutil
@@ -162,7 +163,7 @@ def dump_db() -> bool:
     if not DB.exists():
         return False
     try:
-        with sqlite3.connect(f"file:{DB}?mode=ro", uri=True) as conn:
+        with contextlib.closing(sqlite3.connect(f"file:{DB}?mode=ro", uri=True)) as conn:
             ext = _vec0_extension()
             if ext:
                 try:
@@ -232,9 +233,12 @@ def compare(db: Path = DB, dump: Path = DB_SQL) -> str:
     if not db.exists():
         return "no-live"
     try:
-        with sqlite3.connect(f"file:{db}?mode=ro", uri=True) as live_conn:
+        # closing(), not sqlite3's context manager: that one commits without
+        # closing, and restore_db() unlinks the DB right after calling this -
+        # Windows fails that unlink while any handle is still open.
+        with contextlib.closing(sqlite3.connect(f"file:{db}?mode=ro", uri=True)) as live_conn:
             live = _state(live_conn)
-        with sqlite3.connect(":memory:") as dump_conn:
+        with contextlib.closing(sqlite3.connect(":memory:")) as dump_conn:
             dump_conn.executescript(dump.read_text(encoding="utf-8"))
             backup = _state(dump_conn)
     except (sqlite3.Error, OSError):
@@ -287,7 +291,7 @@ def restore_db() -> None:
     for suffix in ("", "-wal", "-shm"):
         Path(str(DB) + suffix).unlink(missing_ok=True)
     try:
-        with sqlite3.connect(DB) as conn:
+        with contextlib.closing(sqlite3.connect(DB)) as conn, conn:
             conn.executescript(DB_SQL.read_text(encoding="utf-8"))
         print("Memory DB restored (conversations + history + facts).")
     except sqlite3.Error as exc:

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.