docs: artifact panel & agent file return design doc

[baryhuang]

Problem: Agent responses never include file attachments in thread messages.

1. Agent uses workspace_write_file MCP tool — file uploads but response message has no attachments. File appears in Files panel only.
2. Agent uses local Write tool — file saved to disk, never reaches workspace storage. User cannot see it.
3. Thread UI treats only images and HTML as previewable. Markdown/code/text show as download links.

Decisions

• Auto-upload on local Write: When Claude adapter sees a Write tool_use in the CLI stream, it reads file_path and content and uploads to /v1/files/base64. No prompt changes needed.
• Collect uploaded files at response time: Before _send_response(), adapter queries GET /v1/files?channel_name=X&uploaded_by=openagents:Y for files this agent uploaded. Attaches them to the response.
• Dedup via _attached_file_ids: Files already attached in previous responses are skipped.

Changes

• sdk/src/openagents/adapters/base.py — _channel_uploaded_files tracking, track_uploaded_file(), _send_response() passes attachments
• sdk/src/openagents/adapters/claude.py — _upload_local_file() on Write tool_use, _collect_uploaded_files() queries backend
• sdk/src/openagents/client/workspace_client.py — channel_name and uploaded_by params on list_files()
• workspace/backend/app/routers/files.py — channel_name and uploaded_by filters on GET /v1/files
• packages/go and workspace/frontend chat-message.tsx — isPreviewable() expanded for markdown/text/code

Test

1. pip install -e . && start backend on :8000 && frontend on :3001
2. Create agent: openagents create claude --name test-bot --join-workspace TOKEN --endpoint http://localhost:8000
3. Send: create a markdown file about X
4. Agent Write tool fires -> adapter auto-uploads -> response shows eye icon attachment
5. Click -> file preview renders markdown

Not in scope

• Artifact side panel (click still navigates to files view)
• Inline artifact detection from message text
• Orchestrator path (SimpleAutoAgent) not wired

[vercel]

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[zomux]

Merged into develop via 2354bc9. All 3 commits cherry-picked cleanly — no adjustments needed. Thanks @baryhuang!