[FEATURE] Single-key model cycling (Cmd+Tab-style rotation through pinned models)

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Model switching mid-session is increasingly common as teams adopt model-specific workflows (Opus for architecture decisions, Sonnet for implementation, Haiku for quick lookups). But the current options all break flow:

• Alt+P opens a picker overlay -- you still have to visually scan and select. It's two steps.
• /model sonnet only works from an empty prompt. If you're mid-thought composing a prompt and realize the task needs a different model, you have to cancel, switch, and retype.

The mode cycling UX already solves an analogous problem well: Shift+Tab instantly rotates through modes without opening any picker or leaving your prompt. Model switching deserves the same treatment.

Proposed Solution

Add a chat:modelCycle keybinding action that rotates through a user-configured list of pinned models on each keypress. No picker, no menu, just instant switching.

Behavior: Each keypress advances to the next model in the list and wraps around:

Opus → Sonnet → Haiku → Opus → ...

The current model is already visible in the status line, so visual feedback is already in place.

Configuration (in settings.json or similar):

{
  "modelCycle": ["opus", "sonnet", "haiku"]
}

Keybinding (in keybindings.json):

{
  "context": "Chat",
  "bindings": {
    "alt+m": "chat:modelCycle"
  }
}

Alternative Solutions

Why This Is Different From Existing Requests

Alt+P (#7388, resolved) opens a picker — it's two steps, not one.
#31545 asks for a keybinding that opens a picker or cycles models while composing, but doesn't propose a configurable pinned-model list.
#12844 (closed as duplicate) asks for slash commands to switch models, which is a different interaction pattern.
#31519 requests the same cycling UX for effort levels .. this is the model equivalent.

The key differentiator: this request is for a zero-UI, single-keypress rotation through a user-defined subset of models, not a faster way to open the existing picker.

Priority

High - Significant impact on productivity

Feature Category

CLI commands and flags

Use Case Example

Use Case

Our team recommends using different models depending on the task. In a typical session I might:

1. Start with Sonnet for scaffolding and boilerplate
2. Switch to Opus when I hit a tricky architectural decision
3. Drop to Haiku for a quick "what does this flag do?" question
4. Back to Sonnet for implementation

With the current picker flow, each switch requires opening a menu and selecting.. it breaks flow. A single-key cycle (like Cmd+Tab for apps on macOS) would make this seamless.

Additional Context

_No response_