Support ACP and a standardized web UI

[ptone]

Problem / Motivation

Currently the interactivity with the supported harnesses is via a direct connection via web terminal to the underlying TUI (terminal UI) - this makes it easy to universally support any harness designed to work in a shell.

However, the desktop apps for Claude and Codex and ... Support a richer UX with a GUI

This feature tracks the potential use of the ACP protocol as a common, but not universal, way to support a standardized way of adding a richer GUI experience option for Scion agents.

Proposed Solution

Integrate a normalized ACP adapter and build an alternative "attach" method for agents

Alternatives Considered

A2A

Additional Context

#184

https://agentclientprotocol.com/get-started/introduction