Salvage of #15781 by @helix4u onto current main.

Summary

hermes doctor no longer fails config validation for model.provider: custom. Previously listed custom as a known provider and then rejected it as unknown in the same check.

Root cause

known_providers includes bare custom, but _resolve_provider_full("custom", ...) only handles named entries (custom:neuralwatt) and returns None, tripping the fail branch.

Fix

Skip the resolver for custom like we already do for auto. 5 lines in hermes_cli/doctor.py + regression test.

Validation

scripts/run_tests.sh tests/hermes_cli/test_doctor.py — 23/23 passed.

Credit: @helix4u (authorship preserved via cherry-pick). Closes #15781.