feat: add Claude Opus 4.6 (fast mode) to GitHub Copilot provider

[mugnimaestra]

Summary

Adds the Claude Opus 4.6 (fast mode) (preview) model to the github-copilot provider.

This was announced on Feb 7, 2026 and is currently rolling out as a research preview for Copilot Pro+ and Enterprise users.

• Announcement: https://github.blog/changelog/2026-02-07-claude-opus-4-6-fast-is-now-in-public-preview-for-github-copilot/
• Supported models docs: https://docs.github.com/copilot/reference/ai-models/supported-models
• Related issue: Add Claude Opus 4.6 (fast mode) to GitHub Copilot provider #833

Notes & Caveats

I want to be transparent about a few uncertainties:

• Model ID: I've used claude-opus-4.6-fast as the filename/ID, following the pattern of grok-code-fast-1 already in this provider. However, I don't have access to the exact internal model ID that the Copilot API uses — this model is only visible to Pro+/Enterprise users in the model picker. If the maintainers know the correct ID, feel free to rename the file.
• Capabilities: I've mirrored the values from the existing claude-opus-4.6.toml since GitHub describes the fast mode as delivering "the same intelligence as Opus 4.6" but with up to 2.5x faster output token speeds.
• Status: This is a public preview model. I noticed other preview models in this provider (like gemini-3-flash-preview.toml) don't use the status field, so I've followed that pattern. Happy to add status = "beta" if preferred.
• Premium multiplier: 9x during the promotional period (through Feb 16, 2026). Not sure if this is tracked in the TOML schema.

Please let me know if anything needs adjusting — happy to update! 🙏