feat(core): let a bot share a file from its sandbox in its reply

[davemooreuws]

Seventh of the chat attachments PRs, stacked on #421: a bot can show people a file it made in the sandbox, such as a chart or an edited image, under its reply.

• A new sandbox tool, share_file({ path, name? }), offered in a chat's own thread. It reads the file as the agents' user, up to 10 MB and 20 files per reply, and attaches it to the reply being written. The tool tells the bot never to paste contents, base64 or links instead.
• Shared files are kept as the thread_file kind agent_attachment, which feat(core): upload files to a chat, and serve them back #393's migration allows, and shown the moment they're attached, through a new message.files_changed event. The web keeps them on a reply that's still being written.
• A reply with only files counts as an answer. A reply that starts again drops the files it shared, and a turn that has ended refuses new ones.
• A bot's own message doesn't list its files again, since its share_file calls record them.

Files are read as base64 through a command, which caps them at 10 MB. Streaming them out instead is tracked in SUGA-2004.

Part of SUGA-1825

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	sugabots-website	e9266a6	Oct 16 2026, 06:45 AM

[tjholm]

This is 100% one of the features I am after btw so very excited to see this