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.