[Feature Request] Add planModel setting to automatically switch models between plan and execute modes

Bug Description
I want to plan in Opus (deep reasoning) but execute in Sonnet (faster, cheaper).
Today the only workaround is manually running /model sonnet after exiting plan mode, but the
switch is noticeably delayed and easy to forget — so I end up burning Opus tokens on
mechanical edits.

Proposed: A planModel key in settings.json that overrides model while plan mode is active and
automatically reverts on exit. Example:

  {                                                             
    "model": "claude-sonnet-4-6",
    "planModel": "claude-opus-4-6",
    "effortLevel": "high"                                                                      
  }

Why it matters: Plan mode and execution mode have genuinely different cost/quality tradeoffs.
Planning benefits from the strongest reasoning model; execution benefits from speed and
throughput. A native split would make the Opus → Sonnet handoff zero-friction and let teams
standardize the pattern across a project without relying on each user to remember /model
toggles.

Bonus: a matching planEffortLevel would let people pair opus + high for planning with sonnet

• medium for execution.

Environment Info

• Platform: darwin
• Terminal: vscode
• Version: 2.1.105
• Feedback ID: 7d533c5b-0dd3-41c1-a8b9-7f2c80b5c5fe