Add Claude Opus 4.6 (fast mode) to GitHub Copilot provider

Summary

Claude Opus 4.6 fast mode is now available as a public preview in GitHub Copilot (announced Feb 7, 2026), but it's not yet listed in models.dev under the github-copilot provider.

GitHub blog announcement: https://github.blog/changelog/2026-02-07-claude-opus-4-6-fast-is-now-in-public-preview-for-github-copilot/

GitHub Docs (supported models): https://docs.github.com/copilot/reference/ai-models/supported-models

Details from GitHub Docs

From the announcement: "Fast mode for Claude Opus 4.6 delivers output token speeds up to 2.5x faster, while maintaining the same intelligence as Opus 4.6."

Notes

• The existing claude-opus-4.6.toml covers the regular version
• The fast mode variant would likely need a separate entry (e.g. claude-opus-4.6-fast.toml)
• I'm not sure of the exact internal model ID that Copilot uses for this variant — the docs only list the display name
• A status = "beta" field would be appropriate since it's a public preview

Start with the existing claude-opus-4.6.toml entry and compare the GitHub blog announcement with the supported-models documentation. Determine the internal model ID for the fast-mode variant, then add its separate entry with the documented display name and beta status; done means it is listed under the github-copilot provider.