fix(tui): save clipboard-pasted images to temp files for MCP tool access

[mikwiseman]

Issue for this PR

Closes #14673

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

When an image is pasted from the clipboard (Cmd+V / Ctrl+V), OpenCode stores it as an inline data:image/...;base64,... URL. This works for multimodal models, but non-vision models that rely on MCP tools to analyze images break because MCP tools expect a file path on disk, not a raw base64 string.

This PR saves clipboard-pasted images to a temp file in os.tmpdir() and sets the attachment URL to file://..., so MCP tools can read them. The data: URL is kept as fallback if writing fails.

Single file change, ~15 lines added.

How did you verify your code works?

1. bun run typecheck passes across all 13 packages
2. Built a local binary (bun run script/build.ts) and verified:
   • Pasting an image from clipboard creates a file at /tmp/opencode-paste-<timestamp>.png
   • The attachment URL is now a file:// URL instead of data:...
   • An MCP tool (custom zai-vision server reading from the temp path) successfully analyzed the pasted screenshot
3. Verified that non-image attachments are unaffected (no change in behavior)

Screenshots / recordings

No UI changes — purely internal data handling.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR