Add MiniMax provider presets to OpenCode config

[octo-patch]

Reason: Cloud Code's seeded OpenCode configuration does not expose MiniMax models or regional endpoints.

• Add environment-authenticated MiniMax presets for the global and China endpoints across both supported compatible API styles.
• Register MiniMax-M3 and MiniMax-M2.7 with current context limits, pricing, input modalities, and thinking behavior.

Checks:

• pnpm lint
• pnpm exec oxfmt --check config/opencode/opencode.json
• jq -e . config/opencode/opencode.json
• git diff --check origin/master...HEAD

Note: pnpm fmt:check also reports pre-existing formatting issues in unchanged AGENTS.md and README.md files.