SessionManager.save() previously used bare open("w") which could
truncate the JSONL file if the process crashed mid-write. Now writes
to a .tmp file and atomically replaces via os.replace(), matching the
pattern already used in qq.py.
_load() now attempts _repair() before returning None, recovering
valid lines from partially-written files. 12 new tests cover atomic
save correctness, temp-file cleanup on failure, and repair of
truncated/corrupt JSONL.
cowork-with:opencode(glm-5.1)
