Task tool subagent spawning ignores model context window variant [1m]

Bug Description

When spawning teammates/subagents via the Task tool, the [1m] context window variant of the model ID is stripped, resulting in subagents always running on the standard context window version.

Steps to Reproduce

1. Set the model to claude-opus-4-6[1m] via any of:

• CLAUDE_CODE_SUBAGENT_MODEL="claude-opus-4-6[1m]"
• ANTHROPIC_MODEL="claude-opus-4-6[1m]"
• Claude config file with explicit model setting

1. Spawn a teammate via the Task tool
2. Ask the teammate to report its model ID

Expected Behavior

The subagent should report running on claude-opus-4-6[1m] (1M context window variant).

Actual Behavior

The subagent reports running on claude-opus-4-6 (standard context window).

Observed behavior:

• When model param is not specified on the Task tool (inherit from parent): the CLI passes --model claude-opus-4-6 — stripping the [1m] suffix
• When model: "opus" is specified: the CLI passes --model opus — just the enum string, no context window variant

Neither path preserves the [1m] context window specifier regardless of environment variables or config file settings.

Expected Fix

The Task tool's subagent spawning should respect the full model ID including context window specifiers like [1m] from the parent session, environment variables, or config.

Environment

• Claude Code CLI
• Model: claude-opus-4-6[1m]
• Platform: macOS (Darwin 25.1.0)