feat: add configurable cursor style for TUI textareas #11305

Problem

Currently, the cursor style in OpenCode TUI is hardcoded. Users cannot customize the cursor appearance (block, line, underline) or control whether it blinks.

The underlying @opentui/core library already supports CursorStyle type ("block" | "line" | "underline") and CursorStyleOptions ({ style: CursorStyle, blinking: boolean }), but OpenCode doesn't expose this as a configurable option.

Proposed Solution

Add two new configuration options to the tui section of opencode.json:

• cursor_style: "block" | "line" | "underline" (default: "block")
• cursor_blink: boolean (default: true)

Example Configuration

{
  "tui": {
    "cursor_style": "underline",
    "cursor_blink": false
  }
}

Implementation

Apply the cursor style settings to all textarea components:

• Main prompt textarea
• Question textarea
• Permission reject prompt textarea
• Dialog prompt textarea
• Export options textarea

Use Case

Users who prefer a stable underline cursor (common in many terminal applications) instead of the default blinking block cursor.