Offer OpenCode Go in the settings window

A key row under Keys, a model box in the cleanup tab and a box in the
agent tab, each with its own model list of the models OpenCode Go
serves over /chat/completions. Fetch model list reads whichever cleanup
provider is on screen, and the Turkish strings cover the new rows.

tests/test_ui.py

@@ -244,6 +244,7 @@ class Settings(DikteTest):
         """An OpenRouter id and a Claude alias are not the same field."""
         window = self.window(cfg.Config())
         boxes = {"openrouter": window.cleanup_model_row,
+                 "opencode": window.cleanup_opencode_model,
                  "claude": window.cleanup_claude_model,
                  "codex": window.cleanup_codex_model}
         for provider, box in boxes.items():