fix: nightly hardening - CLI config load guards

[mouse-value-add]

problem

PromptLens run assumes config YAML always parses into a top-level object. Empty files or non-object YAML (like lists) can trigger confusing downstream errors during override/parsing.

approach

• Added _load_config_data() in promptlens/cli.py to centralize config loading.
• Added explicit guards to fail early when YAML is empty or top-level is not a mapping.
• Wired run to use this loader before applying CLI overrides.
• Added focused tests for empty, non-mapping, and valid mapping config files.

verification

• Ran: python3 -m pytest -q tests/test_cli_config_loading.py tests/test_cli_hardening.py
• Result: 6 passed.

risks

• Very low risk. Only affects config load/validation path and improves error clarity.

rollback plan

• Revert commit f2ff9ff to restore previous config loading behavior.