Tasks: Add keyboard shortcut (Cmd/Ctrl + Enter) to submit prompt #21650

Description

Currently there's no keyboard shortcut to submit a task prompt. Users who prefer keyboard-driven workflows need to tab multiple times to reach the submit button.

Proposed Solution

Add Cmd + Enter (macOS) / Ctrl + Enter (Windows/Linux) keyboard shortcut to submit the prompt, similar to Devin's implementation:

This allows keyboard-focused users to submit without reaching for the mouse or tabbing through multiple elements.

Acceptance Criteria

• Cmd + Enter submits the prompt on macOS
• Ctrl + Enter submits the prompt on Windows/Linux
• Consider showing a hint in the UI (e.g., near the submit button) indicating the shortcut

Created on behalf of @david-fraley