Any ACP agent works in Compose day one

[tosinamuda]

User story
As a user of a new ACP-speaking agent, I want to add it to Compose and have chat, tool cards, edit review, and cancel work day one, so that I'm never locked to the bundled agent list.

Acceptance criteria

• An ACP agent is addable via Settings → custom agents (command + args), no code changes
• Streaming chat, tool/file-operation cards, EditGuard review, and Stop all work through the normalized event stream
• At least one real ACP agent verified live in the packaged app
• agent-harness README documents the ACP archetype as supported (ecosystem story)

Sub-tasks

• Close the remaining RunEvent↔ACP mapping gaps in agent-harness
• Compose settings wiring + capability probe for ACP agents
• Live verification + docs (harness README + Compose positioning)