feat(agents): add OpenCode as a first-class ACP runtime

[leesalminen]

Summary

Buzz Desktop's managed agent runtime catalog supports Goose, Claude Code, Codex, and Buzz Agent, but OpenCode is missing even though it already speaks ACP natively over stdio:

opencode acp

Users who want OpenCode-backed agents must configure a custom harness command instead of selecting OpenCode from Agent runtimes.

Why

• OpenCode ships a stable ACP server (opencode acp) — no separate *-acp npm adapter is required (same class of integration as Goose's goose acp / Grok's native ACP path).
• Install path is well-known (curl -fsSL https://opencode.ai/install | bash, binary under ~/.opencode/bin or ~/.local/bin).
• Without catalog registration, Desktop cannot discover, install-hint, or process-sweep OpenCode the way it does other runtimes.
• OpenCode's provider-agnostic model catalog (Anthropic, OpenAI, OpenRouter, local Ollama/vLLM endpoints) surfaces through the existing ACP configOptions model discovery with no extra work.

Proposed solution

Register OpenCode in KNOWN_ACP_RUNTIMES with:

Field	Value
id	opencode
label	OpenCode
commands	opencode
Default args	acp
PATH discovery	include ~/.opencode/bin
Config surface	~/.config/opencode/opencode.json (model, provider, MCP servers)
Auth hint	opencode auth login

Also wire the config bridge (model/provider/MCP from opencode.json), buzz-acp default-arg normalization, docs (buzz-acp README / TESTING), and unit tests.

Acceptance criteria

• OpenCode appears under Desktop → Agent runtimes when opencode is on PATH
• Creating/starting a managed agent with runtime=OpenCode spawns opencode acp via buzz-acp
• buzz-acp models with BUZZ_ACP_AGENT_COMMAND=opencode completes an ACP handshake and lists models
• Config surface reads model/provider/MCP servers from ~/.config/opencode/opencode.json
• Docs describe the OpenCode path alongside Goose/Claude/Codex

Related

• Pattern references: Grok Build ACP (feat(agents): add Grok Build as a first-class ACP runtime #2347), Kimi Code runtime (Add Kimi Code runtime support #2365)
• OpenCode ACP docs: opencode acp / Agent Client Protocol mode