[Feature] Support Opus model for Task tool subagents

Context

When running complex multi-file verification tasks, I want to spawn parallel subagents using Opus for better reasoning. The main agent runs Opus 4.5 successfully, but subagents cannot use Opus at all.

Current behavior

Setting CLAUDE_CODE_SUBAGENT_MODEL=claude-opus-4-5-20251101 or using model: "opus" in the Task tool returns:

API Error: 404 {"type":"error","error":{"type":"not_found_error","message":"model: opus"}}

Additionally, the env var overrides any explicit model: parameter in the Task tool, so there's no way to fall back to sonnet either once the env var is set.

Expected behavior

Users with Opus access should be able to spawn Opus subagents. Explicit model: parameters should take precedence over env vars.

Environment

• Claude Code: 2.0.76
• OS: macOS
• Main agent: Opus 4.5 (works)

Related context

This is distinct from existing issues:

• #9470/#9573 were about Sonnet 404 errors (backend issue, now closed)
• #8932 is about model inheritance not working
• #10993 is about unclear documentation
• #13434 is about wrong defaults

This request is specifically about enabling Opus as a supported model for subagents.