Feature: Add chat:cycleModel action for keybinding to cycle through LLMs

Summary

Add a chat:cycleModel bindable action that cycles through available LLMs (e.g. Haiku → Sonnet → Opus → Haiku), similar to how chat:cycleMode cycles through plan/auto modes.

Motivation

Currently, the only way to switch models via keyboard is:

• cmd+p — opens the model picker UI, then requires additional navigation

There is no way to quickly cycle through models with a single keystroke the way shift+tab cycles through modes.

Proposed Solution

Add a new bindable action chat:cycleModel that cycles through the available LLMs in sequence. Users could then bind it to any key, for example:

{
  "$schema": "https://www.schemastore.org/claude-code-keybindings.json",
  "$docs": "https://code.claude.com/docs/en/keybindings",
  "bindings": [
    {
      "context": "Chat",
      "bindings": {
        "ctrl+shift+m": "chat:cycleModel"
      }
    }
  ]
}

Additional Context

• shift+tab is already taken by chat:cycleMode, so a new action is needed
• Cycle order could follow the model tier: Haiku → Sonnet → Opus → (wrap around)
• Would be consistent with the existing pattern of chat:cycleMode