Add default variant config per agent

[opencode-agent[bot]]

All changes are in place. Let me create a summary of the implementation:

Summary

I've implemented the default variant configuration per agent feature as specified in the issue. Here are the changes made:

Files Modified

1. packages/opencode/src/config/config.ts (lines 481, 497):

   • Added variant field to Config.Agent schema with description
   • Added variant to knownKeys set for proper parsing

2. packages/opencode/src/agent/agent.ts (lines 37, 194):

   • Added variant field to Agent.Info type
   • Added parsing logic to read variant from config during agent state initialization

3. packages/opencode/src/session/llm.ts (lines 86-87):

   • Updated variant resolution to use input.user.variant ?? input.agent.variant as the variant key
   • This implements the priority order: explicit user selection > agent's configured default > none

4. packages/opencode/src/tool/task.ts (line 135):

   • Pass agent.variant to subagent sessions so they use their own configured variant

Usage

Users can now configure a default variant for each agent:

{
  "agent": {
    "build": {
      "model": "anthropic/claude-sonnet-4-5-thinking",
      "variant": "high"
    },
    "explore": {
      "model": "google/gemini-3-flash",
      "variant": "low"
    }
  }
}

Variant Priority Order

1. Explicit user selection (--variant flag or ctrl+t)
2. Agent's configured default variant (agent.variant)
3. None (model's base behavior)

Subagent Behavior

Subagents now use their own configured agent.variant rather than inheriting the parent's --variant override, respecting explicit per-agent configuration.

Closes #7138

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