fix: restore config-saved custom endpoint resolution

[teknium1]

Summary

• honor config-saved custom endpoint base URLs during main runtime resolution when provider=custom
• route auxiliary text/vision/main-provider resolution through the same custom endpoint logic
• add regression tests for config-only custom endpoint setups

Test plan

• source /home/teknium/.hermes/hermes-agent/.venv/bin/activate && python -m pytest tests/test_runtime_provider_resolution.py tests/test_cli_provider_resolution.py tests/test_provider_parity.py -n0 -q
• source /home/teknium/.hermes/hermes-agent/.venv/bin/activate && python -m pytest tests/agent/test_auxiliary_client.py -n0 -q
• export HERMES_HOME=/tmp/hermes-home-custom-config-only && source /home/teknium/.hermes/hermes-agent/.venv/bin/activate && python -c "import cli; from agent.auxiliary_client import get_text_auxiliary_client, get_vision_auxiliary_client; c,m=get_text_auxiliary_client(); print('text', m, getattr(c,'base_url',None)); c2,m2=get_vision_auxiliary_client(); print('vision', m2, getattr(c2,'base_url',None))"