Fix/acp agent proxy remote

[Felipenguim]

📝 Summary

Closes #8531

The ACP agent panel built its WebSocket URL from a hardcoded per-agent port:

ws://<hostname>:3017/message   # 3017 for claude

That assumes the browser can reach the agent directly. Behind a reverse proxy —
where only marimo's own port is routed — the connection never establishes, so
the agent panel is unusable in any hosted or containerized deployment.

Fixing this needs changes in th frontend and bakcend. The existing community patch (phisignet/marimo#1)
had only the frontend half, and its author noted it does not work on its own.
Both were implemented here.

Before: browser → agent (direct, port 3017)
After: browser → marimo server (/acp/<agent_id>) → agent (127.0.0.1:<port>/message)

The connection is now same-origin and only needs the port marimo is already
served on.

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✅ Merge Checklist

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• Documentation has been updated where applicable, including docstrings for API changes.
• Tests have been added for the changes made.

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Project	Deployment	Actions	Updated (UTC)
marimo-docs	Ready	Preview	Aug 28, 2026 4:41pm

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