[Feature Request] Add opusplan[1m] preset for Sonnet 4.6 1M context support

Bug Description
Feature request: opusplan[1m] model preset

Currently, opusplan uses Opus 4.7 in plan mode and standard Sonnet 4.6 (200K context) outside
plan mode. There's no way to configure it to use Sonnet 4.6 with 1M context for the non-plan
phase.

Request: add an opusplan[1m] (or similar) preset that pairs Opus 4.7 in plan mode with Sonnet 4.6
(1M context) in normal mode.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.119
• Feedback ID: 3994c8d0-eef9-4592-a6a8-453dcf8c226a