[Feature]: Support for opencode

Problem / Use Case

HumanLayer only supports Claude Code and Codex backends. OpenCode offers many providers (Anthropic, OpenAI, Copilot, Vertex, OpenRouter, local models) behind one interface.

Proposed Solution

Add OpenCode as a third backend, mirroring the Claude/Codex integration:

1. Daemon provider that launches OpenCode sessions and streams events (sketched in PR #930).
2. humanlayer opencode init to install commands/agents into .opencode/ (sketched in PR #906).
3. UI provider/model selection driven by the user's OpenCode config.

Alternative Solutions Considered

• Generic "advanced providers" (BASE_URL/API_KEY/MODEL_NAME) per #761 — maintainers called it experimental; no per-session switching.
• Custom AIProvider strings (PR #829) — rejected.
• OpenRouter integration (#453) — only covers one provider.

Additional Context

#818 and #761 promised an OpenCode backend; PRs #906/#930 died unmerged. #1078 asks for the same outcome but its OpenCode comment has had no reply since Aug 2026.

Related Issues/PRs: #904, #906, #930, #818, #761, #1078, #1032, #453