TUI transparency not working in WSL2 + WezTerm #21397

 - [x] I have searched existing issues and this is not a duplicate                                                                                                                                                                        
 - [x] I understand this issue needs status:approved before a PR can be opened
                                                                                                                                                                                                                                          
 ### Bug Description                                                                                                                                                                                                                      
 The TUI background is not transparent when running OpenCode in WSL2 with WezTerm on Windows, even when using the `system` theme or a custom theme with `"background": "none"`.

 Other TUI apps respect WezTerm's transparency, but OpenCode shows a solid black/opaque background.

 ### Steps to Reproduce
 1. Configure WezTerm with `window_background_opacity = 0.85` and `text_background_opacity = 0.85`
 2. Run OpenCode in WSL2
 3. Observe that the background is opaque instead of transparent

 ### Expected Behavior
 OpenCode should respect terminal transparency like other TUI apps (vim, nano, etc.)

 ### Actual Behavior
 OpenCode displays a solid black background that blocks the terminal's transparent background

 ### Operating System
 - Windows 11 (host)
 - WSL2 Ubuntu (guest)

 ### Agent / Client
 OpenCode 1.3.17

 ### Shell
 bash

 ### Configuration

 WezTerm config (.wezterm.lua):
 ```lua
 config.window_background_opacity = 0.85
 config.text_background_opacity = 0.85

 OpenCode config (opencode.json):

 {                                                                                                                                                                                                                                   ▄
   "theme": "system"  // also tried custom transparent theme                                                                                                                                                                         █
 }                                                                                                                                                                                                                                   █
                                                                                                                                                                                                                                     █
 Additional Context                                                                                                                                                                                                                  █
 - Tested with theme: "system" - doesn't work                                                                                                                                                                                        █
 - Tested with custom theme setting all backgrounds to "none" - doesn't work                                                                                                                                                         ▀
 - Other TUI apps (vim, htop, etc.) correctly show transparency