Adds an Agents General setting to require Cmd/Ctrl+Enter before sending
chat messages. When enabled, plain Enter inserts a newline in agent chat
inputs while the send button remains available.
The preference is now persisted server-side through
`/api/v2/users/{user}/preferences`, alongside the existing user
preference settings, and is applied to both the create-agent input and
existing chat composer. Storybook and API coverage verify the setting,
keyboard behavior, validation, and persistence.
<details>
<summary>Coder Agents notes</summary>
Generated by Coder Agents from a Slack request. Dogfooded with
agent-browser against the Storybook settings and chat input stories.
</details>
Thomas Kosiewski
5c3b59151e