ACP client runtime: host vibe-acp from the fork (lifecycle, streaming, session mapping) #1710

Part of the Vibe-embedding epic (ACP-hosted vibe-acp as successor to the vibe-direct wire emulation).

Scope

Build the fork's ACP client runtime for vibe-acp:

• Spawn and supervise the vibe-acp process (stdio, JSON-RPC 2.0, ACP protocol v1) per host/workspace, with lifecycle management (start on demand, reap on idle/exit, restart on crash with session state preserved by the agent under ~/.vibe).
• Implement the client side of: initialize, session/new, session/prompt, session/cancel, session/load (agent advertises loadSession: true), session/list / session/fork / session/close.
• Map fork sessions ↔ ACP sessions 1:1; stream session/update notifications (agent message/thought chunks, tool calls and their updates) into the fork's session event model.
• Prompt capabilities the agent advertises: text, image, embeddedContext.

Spike proof this builds on

2026-11-05, laptop amd-6: a 211-line stdlib Python host drove a real turn end-to-end (session 48475654-f48c-f816-04aa-56fd06364bd4, tool call executed, usage in the prompt response). Artifacts: ~/work/spikes/vibe-acp-embed-2026-11-05/ on the laptop.

Acceptance

• A fork session can be created against the runtime and driven through a multi-step tool-using turn with streaming output visible in the fork, indistinguishable in shape from a native provider session.
• Killing the vibe-acp process mid-session and restarting it loses no conversation state (session/load resumes).
• Runtime never touches the vibe-direct path; both coexist.