[FEATURE]: Add cursor_style configuration support in tui.json

Verification

• I searched for existing feature requests.
• I discussed this in Discord (if there's an existing conversation).

Description

OpenCode currently hardcodes terminal escape sequences that force a block cursor shape, ignoring the native terminal cursor configuration (commonly beam/vertical line in Terminal.app, iTerm2, and other emulators). The TUI initialization in @opentui/core sets terminal raw mode and emits default escape sequences that reset user cursor preferences.

Proposed Solution

Expose a cursor_style key in tui.json config that accepts one of three values - beam, underline, or block - and emits the corresponding DECSCUSR escape codes during terminal initialization.

Example config:

{
  "$schema": "https://opencode.ai/tui.json",
  "cursor_style": "beam"
}

Alternative solutions

• A settings popup in the TUI itself.
• Using an environment variable to configure cursor shape.