fix(models): remove stale Gemini 3 Pro references

Keep repo-owned CLI, docs, and test fixtures aligned with current Gemini 3.1 naming while leaving upstream catalog behavior untouched.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

docs/guide/overview.md

@@ -220,7 +220,7 @@ You can override specific agents or categories in your config:
 
 **Different-behavior models**:
 
-- Gemini 3 Pro — excels at visual/frontend tasks
+- Gemini 3.1 Pro — excels at visual/frontend tasks
 - MiniMax M2.5 — fast and smart for utility tasks
 - Grok Code Fast 1 — optimized for code grep/search