feat(runtime): production ACP adapter for opencode and Claude Code #63

Parent

#62

What to build

Turn the spike adapter (src/runtime/acp-adapter.ts) into the production ACP adapter (ADR 0013
§1, §4, §5, §6). In this layer it sits beside the opencode adapter and is not yet the default
(#64 switches the default).

• Agent definitions for claude (bundled @agentclientprotocol/claude-agent-acp) and
  opencode (opencode acp): command, env, session/new _meta, and how host settings are
  excluded. Settings are excluded when asked to (a flag on the definition or factory); the config
  knob that sets it lands in feat(workflow): choose agent and model separately, ACP as the default runtime #64.
• Host-settings isolation (ADR 0013 §3, open item):
  • Claude: _meta.claudeCode.options.settingSources: ["project","local"]. Verify with a real
    prompt whether this keeps the user's skills and ~/.claude/CLAUDE.md out.
  • opencode: find a mechanism that hides ~/.config/opencode without hiding other tools'
    XDG config (gh auth in ~/.config/gh). Check opencode for a config-dir env var first. Record
    the result in finding 13's addendum.
• Environment scrub: drop inherited CLAUDE_* / CLAUDECODE variables from the agent's
  environment.
• Model handling: the adapter requires a model and fails before the prompt when the agent
  does not offer it. The error names the agent and gives a sample of offered ids.
• Headless permission callback: prefer allow_always, then allow_once.
• Usage: forward each ACP usage_update as a CUSTOM acp.usage chunk carrying a new
  usage signal (context: {used, size}, cost: {amount, currency}). Extend the
  AgentSignal union. Recording the signal in events is feat(events): context and cost per agent step, ACP-aware run detail #65, so the runtime may ignore it here.
• Cancel: session/cancel, then kill after a grace period, and kill in finally.
• Process exit mid-turn surfaces as a step error carrying the tail of stderr.
• Availability check: a function the daemon can call to report which configured agents are
  missing (opencode not on PATH, Claude not logged in if detectable).
• Tests: unit tests with a fake ACP agent over stdio (a small script speaking the SDK's
  AgentSideConnection). Cover model selection and rejection, the permission answer, usage
  signal, cancel and crash. Plus one live run per agent via spikes/13-acp-agents.ts.

Acceptance

• bun run check green.
• Live: bun spikes/13-acp-agents.ts run claude sonnet and … run opencode opencode/big-pickle
  complete; cancel leaves no processes.
• Isolation verified for both agents, with the result written into finding 13.