fix(cli): tolerate BOM in ov config

[duyua9]

Summary

• read ov.conf with utf-8-sig in openviking-server doctor
• read ov.conf with utf-8-sig in OpenVikingConfigSingleton._load_from_file
• add focused coverage for BOM-prefixed config files

Validation

• uvx ruff check openviking_cli/doctor.py openviking_cli/utils/config/open_viking_config.py tests/cli/test_doctor.py tests/test_config_loader.py
• uvx ruff format --check openviking_cli/doctor.py openviking_cli/utils/config/open_viking_config.py tests/cli/test_doctor.py tests/test_config_loader.py
• python3 -m py_compile openviking_cli/doctor.py openviking_cli/utils/config/open_viking_config.py tests/cli/test_doctor.py tests/test_config_loader.py
• ad-hoc BOM smoke for check_config() and OpenVikingConfigSingleton.initialize(config_path=...)
• git diff --check

Note: direct pytest collection in this local checkout is blocked by a missing optional test dependency (pathspec) imported from tests/conftest.py, so I used the focused import/behavior smoke above for the new paths.

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