[FEATURE]: Publish exact model versions/checkpoints for Go plan models (e.g. whether deepseek-v4-flash serves 0731)

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Please document the exact model version/checkpoint that each model in the Go plan serves on the official site (https://opencode.ai/go and https://opencode.ai/docs/go/), and keep it up to date when a model is rolled forward.

Background

Today the Go plan pages only list model names like "DeepSeek V4 Flash" with no version information. But DeepSeek updated the same model ID deepseek-v4-flash from the initial preview to a new checkpoint DeepSeek-V4-Flash-0731 (formal version, 2026-07-31) without any public rollout notice from OpenCode's side. Users of opencode-go/deepseek-v4-flash currently have no way to tell which checkpoint they are being served and billed for, and behavior changes silently:

• https://github.com/anomalyco/opencode/issues/39823 — users asking whether Go/Zen already serves the 0731 checkpoint (community-reported answer, not an official statement)
• https://github.com/anomalyco/opencode/issues/40409 — a user verified the Go gateway was still NOT serving 0731 (self-identified as V3.2, knowledge cutoff 2025-05) before it was closed
• https://github.com/anomalyco/opencode/issues/40346 — deepseek-v4-flash-0731 is not even selectable in the model selector
• DeepSeek's own docs say "the deepseek-v4-flash model has been updated to DeepSeek-V4-Flash-0731, calling method remains unchanged" — so version changes are silent by design

Request

1. On the Go plan page / docs, show the exact served version for each model, e.g. DeepSeek V4 Flash — checkpoint DeepSeek-V4-Flash-0731 (released 2026-07-31).
2. Add a version history / changelog per model so silent rollouts become visible (e.g. deepseek-v4-flash: preview → 0731 on 2026-08-XX).
3. Ideally expose the served checkpoint programmatically (model metadata in /models or via the API) so it can be verified with a simple request instead of guesswork.

Benefits

• Transparency: users know exactly what they are paying for, which matters for billing and quality expectations.
• Easier bug triage: version info helps users report issues accurately and helps maintainers repro.
• Avoids the repeated confusion seen in #39823 / #40409 / #40346.