OpenCode agent hardcodes obsolete MiniMax model

[midnattsol]

Bug Description

The OpenCode agent hardcodes opencode/minimax-m2.1-free as the default model, which appears to be obsolete/unavailable. This causes failures when using the opencode agent without explicitly specifying a model.

Location

internal/agent/opencode.go - lines 114 and 131

Root Cause

The model is hardcoded in two places instead of being configurable or omitted entirely (letting OpenCode use its own default).

Suggested Fix

Rather than hardcoding another model that may also become obsolete, the better approach is to not pass --model at all when no model is explicitly configured, allowing OpenCode to use its own default model (which it manages/updates).
This follows the principle of least surprise and avoids roborev needing to track OpenCode's model availability.