docs: sync agent-model-matching guide with actual fallback chains
- Metis: add missing GPT-5.4 high as 2nd fallback
- Hephaestus: add GPT-5.4 (Copilot) fallback, was incorrectly listed as Codex-only
- Oracle: add opencode-go/glm-5 as last fallback
- Momus: add opencode-go/glm-5 fallback, note xhigh variant
- Atlas: add GPT-5.4 medium as 3rd fallback
- Sisyphus: add Kimi K2.5 (moonshot providers) in chain
- Sisyphus-Junior: add missing agent to Utility Runners section
- GPT Family table: merge duplicate GPT-5.4 rows
- Categories: add missing opencode-go intermediate fallbacks for
visual-engineering, ultrabrain, quick, unspecified-low/high, writing
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