fix: replace Bun globals with Node fs APIs for OpenCode 1.18 compatibility

[Tony-ooo]

Problem

On OpenCode 1.18.x the plugin runtime no longer exposes the global Bun object. Both plugins/vision-helper.ts and tools/vision.ts call Bun.file() / Bun.write(), which now throw:

ReferenceError: Bun is not defined

This crashes experimental.chat.messages.transform (logged as prompt_async failed), so pasted images are never saved and the path hint is never injected. Non-vision models then only see the raw Cannot read "image.png" (this model does not support image input) error instead of being guided to the vision tool / subagent.

Fix

Replace all Bun file APIs with standard Node fs APIs:

• plugins/vision-helper.ts: Bun.file().exists() → fs.access(), Bun.write() → fs.writeFile()
• tools/vision.ts: Bun.file() → fs.stat() / fs.readFile(), plus a mimeType() helper to infer MIME from the file extension (replacing Bun.file().type)

Verification

• Pasted images are now saved to the temp dir and the path hint is injected correctly on OpenCode 1.18.18.
• The image-reader subagent delegation path works end-to-end (verified with a pasted image).
• Both files pass node --experimental-strip-types --check.