resolveApiKey returns empty string with no clear error

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When no API key is configured and no env var is set, resolveApiKey() returns an empty string. The downstream API call fails with a confusing HTTP 401 error instead of a clear 'no API key configured' message.\n\nSteps to reproduce:\n- Remove API key from config and env\n- Run commit-echo suggest\n- Receive HTTP 401 error from LLM API\n\nExpected:\n- Show a clear error if no API key is configured before making the API call.\n\nType: Bug