tui: paste/drag-drop image attachments not supported in chat prompt

Summary

The v2 TUI does not support pasting or drag-and-dropping images into the
chat prompt. According to the v2 attachments documentation, "Desktop and
web clients provide Attach file, paste, and drag-and-drop controls for
supported text and image files", but those controls are not surfaced in
the TUI. As a workaround, the screenshot has to be saved to a file in
the working directory and explicitly read by the model.

Environment

• opencode version: opencode2 v0.0.0-next-17444
• OS: Linux 6.18.44 (NixOS, x86_64)
• Terminal: alacritty (TERM=xterm-256color, COLORTERM=truecolor)
• Shell: zsh (/run/current-system/sw/bin/zsh)
• Install/channel: local/next (side-by-side with opencode v1 1.18.21)
• Active plugins: none

Reproduction

1. Copy an image to the clipboard (e.g. via satty, grim, or any
   screenshot tool).
2. Launch opencode2 and open a session.
3. In the chat prompt, paste the image (Ctrl+Shift+V, middle-click,
   etc.) and/or drag-and-drop an image file onto the TUI.
4. Observe that nothing is attached to the outgoing message.

Expected Behavior

Per the v2 attachments documentation, paste and drag-and-drop controls
should be available to attach PNG/JPEG/GIF/WebP images to a prompt so
the model receives them as image media, without needing to save them to
a file first.

Actual Behavior

Both paste and drag-and-drop are silently ignored by the TUI prompt.
The only workaround is to save the screenshot to a file in the working
directory and prompt the model with the file path, e.g. "can you check
the img i put in the dir and tell me what it is?".

Additional Context

• Docs reference: https://opencode.ai/v2/docs/attachments/
• Related quote: "Desktop and web clients provide Attach file, paste,
  and drag-and-drop controls for supported text and image files."
• Many modern terminals expose clipboard images via escape sequences
  (Kitty/iTerm2 image protocol, OSC 52, etc.) or support Sixel/iTerm2
  inline-image queries. Even without inline display, capturing pasted
  image data and persisting it to a temporary attachment file would be
  enough to satisfy the documented contract.
• This was also filed against the opencode v1 TUI in spirit; this
  report is specifically about the v2 (opencode2) binary.