Task tool 'opus' model parameter resolves to claude-opus-4-1 instead of claude-opus-4-6

Bug Description

When spawning subagents via the Task tool with model: "opus", it resolves to claude-opus-4-1 instead of claude-opus-4-6. This causes failures on organizations that have allowlisted claude-opus-4-6 but not claude-opus-4-1 in their Vertex AI organization policy.

Expected Behavior

The "opus" model parameter should resolve to claude-opus-4-6 (the latest Opus model), or should respect the "model": "claude-opus-4-6" setting in ~/.claude/settings.json.

Actual Behavior

API Error: 400 Organization Policy constraint constraints/vertexai.allowedModels violated 
for projects/XXXXX attempting to use a disallowed Gen AI model claude-opus-4-1. 
Please contact your organization administrator to fix this violation by adding 
publishers/anthropic/models/claude-opus-4-1:predict to the allowed values.

Environment

• Claude Code main session model: claude-opus-4-6 (set in ~/.claude/settings.json)
• Platform: macOS (Darwin 25.2.0)
• Provider: Vertex AI (Google Cloud)
• Org policy: constraints/vertexai.allowedModels allows claude-opus-4-6 but not claude-opus-4-1

Steps to Reproduce

1. Set "model": "claude-opus-4-6" in ~/.claude/settings.json
2. Have org policy that allows claude-opus-4-6 but blocks claude-opus-4-1
3. Use Task tool with model: "opus" parameter
4. Subagent fails with 400 error referencing claude-opus-4-1

Suggested Fix

Either:

1. Have the "opus" enum resolve to claude-opus-4-6 (the current latest)
2. Inherit the parent session's model when model: "opus" is specified and the parent is already running opus
3. Support a configuration option (env var or settings.json) to control which specific model ID the "opus" alias maps to for subagents