tui: image paste fails silently on WSL when no clipboard tool is installed (V2)

Summary

opencode V2 on WSL2: pasting an image (Ctrl+V) does nothing when the Linux
clipboard tools (wl-clipboard/xclip) are not installed. No error, no hint.
opencode should either read the Windows clipboard directly (zero Linux
dependencies) or show a clear install message.

Reproduction

1. WSL2 without wl-paste/xclip (default state).
2. Install opencode V2: curl -fsSL https://opencode.ai/v2/install | bash.
3. On Windows, copy an image (e.g. Win+Shift+S screenshot).
4. In the TUI (Windows Terminal), press Ctrl+V.
5. Result: nothing happens. No attachment, no error message.

Windows Terminal (1.24.11911.0) sends an empty bracketed paste for
image-only clipboards (microsoft/terminal #20037), so the TUI reaches
prompt.paste -> clipboard.read(), which returns undefined silently.

Root cause (what to look for)

packages/tui/src/clipboard.ts read() on Linux tries, in order:
wl-paste -t image/png -> xclip -t image/png -o -> clipboardy.
When the first two are missing, the clipboardy fallback fails silently in
standalone binaries (same root cause as #13800 / PR #48611), and
prompt.paste treats undefined as "nothing to paste" with no feedback.

Proposed solutions

1. WSL: read the Windows clipboard via powershell.exe interop first
   (when os.release() contains "WSL"). Requires no Linux packages; verified
   working on the reporter's machine.
2. read() should report failure instead of returning undefined, so the TUI
   can show an actionable message (e.g. sudo apt install xclip wl-clipboard).
3. Accept image/bmp from wl-paste: WSLg syncs the image as image/bmp,
   so requesting only image/png misses it.
4. Optional: offer the one-line install command (never install silently).

Environment

• opencode v2.0.8 (V2 standalone binary, curl installer)
• WSL2, kernel 6.18.33.1-microsoft-standard-WSL2 (Windows 11)
• Windows Terminal 1.24.11911.0, bash, no plugins

Start with packages/tui/src/clipboard.ts and trace read() through prompt.paste, focusing on the Linux tool fallbacks and the undefined result. Compare the WSL2 clipboard behavior and proposed approaches, then define completion as either successful image attachment without Linux clipboard packages or an actionable failure message.