Add MiniMax M3 Free model to opencode/zen

[bitsnaps]

Summary

Tested today, model id: minimax-m3-free and listed as MiniMax M3 Free at opencode.

[jatmn]

I found a couple of issues that need to be addressed before this is ready.

Findings

• [P1] Add the missing OpenCode model descriptor
  src/integrations/gateways/opencode.ts:75
  This new catalog entry points modelDescriptorId at opencode-minimax-m3-free, but there is no matching descriptor in src/integrations/models/opencode.ts. Even with the separate native MiniMax M3 descriptor present, that descriptor is minimax-m3, so it does not satisfy this OpenCode-specific reference. The registry rejects the dangling reference, and the focused integration tests fail with opencode:minimax-m3-free:opencode-minimax-m3-free as a missing model descriptor. It also means descriptor-backed helpers such as getModelsForGateway('opencode') cannot enrich or expose metadata for the new model. Please add the matching defineModel entry (with the right context/output/capability metadata) or point this catalog entry at an existing descriptor that accurately represents the gateway model.

• [P2] Update the OpenCode Zen model count metadata
  src/integrations/gateways/opencode.ts:23
  Adding minimax-m3-free increases the Zen catalog from 41 to 42 models, but the preset description still says 41 models, and the same count is documented in README.md and docs/advanced-setup.md. The OpenCode gateway tests also still assert 41, so after the descriptor is added the user-facing provider metadata and docs will remain stale unless these counts are updated together.

[bitsnaps]

PR #1498