feat(github-copilot): add Claude Opus 4.6 fast mode model

[zhusidao]

Claude Opus 4.6 (fast mode) is available in GitHub Copilot
(30x multiplier, Preview status) but is missing from the
github-copilot provider in models.dev.

This causes OpenCode to not display this model in the /models list.

The model is confirmed available via gh copilot CLI model selection.

Suggested file: providers/github-copilot/models/claude-opus-4-6-fast.toml

Ref: https://docs.github.com/en/copilot/reference/ai-models/supported-models