chore: update GPT-5.6 pricing

[opencode-agent]

Summary

• update direct OpenAI and OpenCode Zen pricing for GPT-5.6 Terra and Luna
• update the direct OpenAI fast-mode prices for Terra and Luna
• add the gpt-5.6-sol-fast model to the OpenCode provider

Source

• https://developers.openai.com/api/docs/pricing
• https://x.com/OpenAI/status/2082878156483219672

Checks

• bun validate
• git diff --check

Requested by: @fwang (Frank via Slack)

[github-actions]

Action items

• [low] [possible mistake] providers/opencode/models/gpt-5.6-sol-fast.toml:1 - Check: Data-changing PRs should cite first-party sources mapped to each material claim. Why: The new OpenCode model ID gpt-5.6-sol-fast (and its availability on Zen) is a material catalog fact, but the PR only cites OpenAI pricing/tweet sources. Those can support the matching fast-tier dollar amounts, not that OpenCode exposes this model id. Action: Add an OpenCode Zen models/pricing/docs citation in the PR body stating it supports gpt-5.6-sol-fast availability and pricing, or confirm the exact Zen model id if it differs.

[github-actions]

No actionable findings.

[MojangPlsFix]

Don't know if out of scope for this PR, but Github Copilot also uses these adjusted prices.