GPT-5.3 Codex has strong multimodal capabilities. Promote it to first
candidate in multimodal-looker fallback chain, with gemini-3-flash
following (matching the ULW pattern of gpt-5.3-codex -> gemini).
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
YeonGyu-Kim