feat(agents): add OpenCode as a native ACP runtime

[leesalminen]

Summary

Closes #2368.

• Registers OpenCode in Buzz's managed ACP runtime catalog (id: opencode) so Desktop can discover and spawn it like Goose/Claude/Codex.
• Uses OpenCode's native ACP entrypoint (opencode acp) — no separate *-acp npm adapter.
• Adds a config bridge for ~/.config/opencode/opencode.json (model, provider from the provider/model string, MCP servers with enabled flags), ~/.opencode/bin binary discovery, process-sweep name opencode, docs, and unit tests.

Why

OpenCode already speaks ACP over stdio (opencode acp). Users currently cannot select it as a first-class Buzz harness without a custom command. This mirrors the Grok Build (#2344) and Kimi Code (#2365) runtime registration pattern. OpenCode's provider-agnostic catalog (Anthropic, OpenAI, OpenRouter, local Ollama/vLLM) surfaces through the existing ACP configOptions model discovery unchanged.

What changed

Area	Change
KNOWN_ACP_RUNTIMES	New OpenCode entry (CLI-login runtime, opencode auth login hint)
Default args	acp — in both desktop discovery and buzz-acp arg normalization
Config bridge	~/.config/opencode/opencode.json → model / provider / MCP servers ($OPENCODE_CONFIG override honored)
PATH	Discovers ~/.opencode/bin
Persona UI	opencode classified as CLI-login / provider-locked, like claude/codex
Process sweep	opencode added to KNOWN_AGENT_BINARIES
Docs	README / TESTING / buzz-acp README
Tests	discovery avatar + arg normalization (desktop + buzz-acp), config bridge parsing, persona UI capability

Validation

• just ci — all recipes green (fmt, clippy, desktop lint + tests + build, tauri check + tests, web build, mobile tests) except one pre-existing buzz-db unit-test failure that reproduces identically on a clean main checkout on Linux — filed as test(buzz-db): replica_fence unit test fails on Linux due to sub-microsecond Utc::now() precision #2369, unrelated to this change
• cargo test -p buzz-acp --lib normalizes_ — 6 passed (incl. OpenCode)
• cargo test --manifest-path desktop/src-tauri/Cargo.toml managed_agents — passed
• Live ACP handshake: BUZZ_ACP_AGENT_COMMAND=opencode BUZZ_ACP_AGENT_ARGS=acp buzz-acp models --json returns the full model list (configOptions, category model) in ~3s
• Manual dev launch (just desktop-standalone): OpenCode detected from PATH, agent created with runtime=OpenCode, live model picker populated via ACP discovery against a local relay

How to try

curl -fsSL https://opencode.ai/install | bash
opencode auth login

export BUZZ_PRIVATE_KEY=nsec1...
export BUZZ_RELAY_URL=wss://your-relay
export BUZZ_ACP_AGENT_COMMAND=opencode
buzz-acp

Then @mention the agent from Buzz Desktop (owner-only by default).

Checklist

• New behavior has tests
• User-facing setup is documented
• No new unwrap() in production code
• No unsafe code

🤖 Generated with Claude Code