fix: remove github-copilot association from gpt-5-nano model mapping

explore agent uses opencode/gpt-5-nano exclusively — github-copilot
should not be associated with gpt-5-nano in docs, tests, or fallback chains.

docs/configurations.md

@@ -404,7 +404,7 @@ Each agent has a defined provider priority chain. The system tries providers in
 | **Sisyphus** | `claude-opus-4-5` | anthropic → github-copilot → opencode → antigravity → google |
 | **oracle** | `gpt-5.2` | openai → anthropic → google → github-copilot → opencode |
 | **librarian** | `big-pickle` | opencode → github-copilot → anthropic |
-| **explore** | `gpt-5-nano` | opencode → anthropic → github-copilot |
+| **explore** | `gpt-5-nano` | anthropic → opencode |
 | **multimodal-looker** | `gemini-3-flash` | google → openai → zai-coding-plan → anthropic → opencode |
 | **Prometheus (Planner)** | `claude-opus-4-5` | anthropic → github-copilot → opencode → antigravity → google |
 | **Metis (Plan Consultant)** | `claude-sonnet-4-5` | anthropic → github-copilot → opencode → antigravity → google |

docs/guide/installation.md

@@ -183,7 +183,7 @@ When GitHub Copilot is the best available provider, oh-my-opencode uses these mo
 | ------------- | -------------------------------- |
 | **Sisyphus**  | `github-copilot/claude-opus-4.5` |
 | **Oracle**    | `github-copilot/gpt-5.2`         |
-| **Explore**   | `github-copilot/gpt-5-nano-fast-1`|
+| **Explore**   | `opencode/gpt-5-nano`              |
 | **Librarian** | `zai-coding-plan/glm-4.7` (if Z.ai available) or fallback |
 
 GitHub Copilot acts as a proxy provider, routing requests to underlying models based on your subscription.