feat(plugin): add chat.variant hook for programmatic thinking/variant control

[code-yeongyu]

Summary

Add chat.variant plugin hook that allows plugins to programmatically control the thinking/variant level (high, max) before LLM requests are made.

Changes

• Add chat.variant hook type definition in @opencode/plugin
• Trigger hook in llm.ts before variant options are applied
• Plugin receives current variant state and can override via output.variant or output.options

Use Case

Plugins can now programmatically control:

• Anthropic's thinking budget (high/max)
• OpenAI's reasoning effort
• Other provider-specific variant options

This enables use cases like:

• Always use high thinking for specific models
• Dynamic thinking level based on conversation context
• Conditional variant selection based on session/agent type

Example Usage

const myPlugin: Plugin = async () => ({
  "chat.variant": async (input, output) => {
    // Always use high thinking for Claude
    if (input.model.id.includes("claude")) {
      output.variant = "high"
    }
  }
})

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