opusplan: system prompt reports sonnet-4-6 in plan mode instead of opus-4-6

Bug Description

When using the opusplan model configuration and entering plan mode (Shift+Tab), the system prompt incorrectly identifies the model as Sonnet 4.6 instead of Opus 4.6.

Steps to Reproduce

1. Set model to opusplan via /model command
2. Enter plan mode (Shift+Tab)
3. Observe the system prompt

Expected Behavior

In plan mode, the system prompt should report:

"You are powered by the model named Opus 4.6. The exact model ID is claude-opus-4-6."

Actual Behavior

The system prompt reports:

"You are powered by the model named Sonnet 4.6. The exact model ID is claude-sonnet-4-6."

The /model command output also shows opusplan (claude-sonnet-4-6) — it's unclear whether this is just a display issue or if Sonnet is actually being used in plan mode instead of Opus.

Impact

• Users cannot confirm whether Opus is actually being used during plan mode
• If the underlying model is also Sonnet (not just the display), then opusplan is not functioning as designed — users are paying for and expecting Opus-level reasoning during planning but getting Sonnet instead

Environment

• Claude Code (latest version as of 2026-03-12)
• macOS (Darwin 25.3.0)