Option to swap Enter and Cmd+Enter in the composer (Enter = newline, Cmd+Enter = send)

### Feature request

Let me swap the two composer keys: make plain **Enter** do what **Cmd/Ctrl+Enter** does today, and make **Cmd/Ctrl+Enter** send the message.

**Current behavior**

- `Enter` — sends the prompt to the session immediately.
- `Cmd/Ctrl+Enter` — inserts a newline / continues editing.

**Requested behavior (as an option)**

- `Enter` — newline, keep editing.
- `Cmd/Ctrl+Enter` — send.

**Why**

Long prompts are the normal case, not the exception: multi-line instructions, pasted logs, file paths, numbered requirements. With `Enter` bound to send, every half-written prompt gets fired off by muscle memory, and the only way to recover is to interrupt the agent and retype. Swapping it makes the destructive action (send) the deliberate one and the frequent action (newline) the cheap one. This is the convention in Slack's "Cmd+Enter to send" mode, many IDE chat panes, and several other agent UIs.

**Shape of the ask**

- A setting, not a hard change — some people prefer `Enter` to send.
- Something like `"composerSubmitKey": "enter" | "mod+enter"` in settings.json, plus a toggle in `/config`.
- Applies to both the CLI composer and the desktop app Code tab, so muscle memory carries across.
- The hint text under the input should reflect the active binding.

Start by tracing the CLI composer and desktop app Code tab, then inspect the settings.json and /config entry points named in the request. Verify the selected binding in both surfaces and that the input hint reflects it; completion should preserve the existing default while supporting both requested modes.

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