gpt-5.3-codex is not available on GitHub Copilot. The fallback chains
incorrectly listed github-copilot as a valid provider for this model,
causing the doctor to report 'configured model github-copilot/gpt-5.3-codex
is not valid' for Hephaestus agent.
Affected agents: hephaestus (requiresProvider + fallbackChain)
Affected categories: ultrabrain, deep, unspecified-low
Copilot users can still use Hephaestus via openai or opencode providers.
Fixes#2047
