docs: fix zen-free observer model to image-capable mimo-v2.5-free (#690)

north-mini-code-free lacks image support; observer does visual analysis, so
use mimo-v2.5-free which has it.

docs/opencode-zen-free-preset.md

@@ -53,7 +53,7 @@ You need an API key for the `opencode` provider. Sign up at [OpenCode Zen](https
         "mcps": []
       },
       "observer": {
-        "model": "opencode/north-mini-code-free",
+        "model": "opencode/mimo-v2.5-free",
         "temperature": 0.2,
         "variant": "low",
         "skills": [],