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: