feat: require double Ctrl+C to exit when input is empty

[noah79]

Summary

• Prevents accidental exits by requiring users to press Ctrl+C twice within 3 seconds when the prompt input is empty
• Shows a toast warning on the first Ctrl+C press: "Press Ctrl+C again to exit"
• Adds Undo Support: Press Cmd+Z (or Ctrl+-) to restore input that was cleared by Ctrl+C
• Matches Claude Code behavior for a more forgiving exit experience

Changes

• Add exitOnCtrlC: false to render options to prevent opentui from handling Ctrl+C directly
• Track Ctrl+C press state in Prompt component with 3-second reset timeout
• Show warning toast on first Ctrl+C with empty input
• Exit on second Ctrl+C within timeout window
• Save cleared state when input_clear (Ctrl+C on text) is triggered
• Restore state on input_undo keybind if a cleared state exists
• Add empty SIGINT handler to prevent default process termination

Files Modified

• packages/opencode/src/cli/cmd/tui/app.tsx - Disable default Ctrl+C handling
• packages/opencode/src/cli/cmd/tui/component/prompt/index.tsx - Double Ctrl+C and Undo logic
• packages/opencode/src/index.ts - Empty SIGINT handler

[noah79]

[coulsontl]

+1，this feature is greatly needed.