feat(coding-agent): added designer role with model fallback support and Gemini 3.1

- Added designer model role for UI/UX design tasks with Gemini 3.1 Pro as default model.
- Implemented model role fallback list support enabling automatic fallback to next available model when primary is unavailable.
- Refactored model role resolution to support multiple fallback patterns per role with thinking level mapping.
- Updated designer agent to use pi/designer role alias instead of explicit model list, removing spawns configuration.
- Added test coverage for model resolver fallback patterns and designer role override preferences.
- Added test coverage for geminiImageTool X-Title header routing through OpenRouter.

packages/coding-agent/src/priority.json

@@ -25,5 +25,13 @@
 		"opus-4.1",
 		"opus-4-1",
 		"pro"
+	],
+	"designer": [
+		"google-gemini-cli/gemini-3.1-pro",
+		"google-gemini-cli/gemini-3-pro",
+		"gemini-3.1-pro",
+		"gemini-3-1-pro",
+		"gemini-3-pro",
+		"gemini-3"
 	]
 }