fix(gateway): use atomic writes for config.yaml to prevent data loss (#3800)
Replace all 5 plain open(config_path, 'w') calls in gateway command handlers with atomic_yaml_write() from utils.py. This uses the established tempfile + fsync + os.replace pattern to ensure config.yaml is never left half-written if the process is killed mid-write. Affected handlers: /personality (clear + set), /sethome, /reasoning (_save_config_key helper), /verbose (tool_progress cycling). Also fixes missing encoding='utf-8' on the /personality clear write. Salvaged from PR #1211 by albatrosjj.
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