Feature: persist default model/agent via /opencode:setup #20

Motivation

Users who consistently prefer a specific provider/model (e.g. anthropic/claude-opus-4-6) or agent (build vs plan) have to pass --model or --agent on every command invocation of /opencode:review, /opencode:adversarial-review, and /opencode:rescue. The setup command already persists state like the review gate toggle via state.config in plugins/opencode/scripts/lib/state.mjs, so it's a natural place to store defaults.

Proposed UX

/opencode:setup --default-model anthropic/claude-opus-4-6 --default-agent build
/opencode:setup  # displays current defaults

Defaults apply to all commands (task, review, adversarial-review) unless overridden per invocation with explicit --model / --agent.

Implementation sketch

• handleSetup in opencode-companion.mjs accepts --default-model / --default-agent and writes them to state.config.defaults.
• handleReview, handleAdversarialReview, handleTask read state.config.defaults and apply them when the caller hasn't passed the flag explicitly.

Upstream reference

Derived from openai/codex-plugin-cc#44.