feat(core): let bots read attached files and see attached images

[davemooreuws]

Third of the chat attachments PRs, stacked on #394: read_thread_file reads what people attach.

• A file is read as text when it was given a text type (in any encoding), or when its bytes are UTF-8 without NULs, so source code typed video/mp2t still reads. Anything else is refused after reading its first chunk.
• PNG, JPEG, GIF and WebP are shown to models that take images, typed by their bytes (detectMediaType from the AI SDK) and capped at 5 MiB encoded. read_file now types its images the same way.
• Images are counted, not kept, when a tool call's output is stored, so later turns' history no longer carries base64.
• Whether a model takes images is now loaded once with the turn's context and shared by the sandbox tools and read_thread_file.
• Through OpenRouter, the images a tool returns go in a user message after its result, with a note where each was. Gemini there refuses a request with an image in a tool result once its reasoning is in the request. Tool images also move from the AI SDK's deprecated image-data part to file.

Part of SUGA-1825

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	sugabots-website	6e91da3	Oct 16 2026, 06:45 AM