fix(memory): eliminate TOCTOU race in Windows file lock creation

On Windows (msvcrt path), _file_lock() first checked if the lock file
existed and wrote it with write_text(), then opened it with open('r+').
Between these two calls, another process could delete the file causing
open('r+') to raise FileNotFoundError — uncaught, leaving memory writes
to proceed without holding the lock, risking data corruption.

Replace the three-line sequence with a single open('a+', ...) call which
atomically creates the file if missing or opens it if it exists, closing
the TOCTOU window entirely. The existing fd.seek(0) before msvcrt.locking()
is preserved and sufficient for correct lock byte positioning.

Root cause: TOCTOU between lock_path.write_text() and open('r+')
Impact: concurrent memory writes on Windows could corrupt MEMORY.md

tools/memory_tool.py

@@ -156,10 +156,7 @@ class MemoryStore:
             yield
             return
 
-        if msvcrt and (not lock_path.exists() or lock_path.stat().st_size == 0):
-            lock_path.write_text(" ", encoding="utf-8")
-
-        fd = open(lock_path, "r+" if msvcrt else "a+", encoding="utf-8")
+        fd = open(lock_path, "a+", encoding="utf-8")
         try:
             if fcntl:
                 fcntl.flock(fd, fcntl.LOCK_EX)