feat(config): Add Fallback Model and Expanded Provider Support

Introduces a configurable fallback model and adds support for additional AI provider API keys in the environment setup.

- **Add Fallback Model Configuration (.taskmasterconfig):**
  - Implemented a new  section in .
  - Configured  as the default fallback model, enhancing resilience if the primary model fails.

- **Update Default Model Configuration (.taskmasterconfig):**
  - Changed the default  model to .
  - Changed the default  model to .

- **Add API Key Examples (assets/env.example):**
  - Added example environment variables for:
    -  (for OpenAI/OpenRouter)
    -  (for Google Gemini)
    -  (for XAI Grok)
  - Included format comments for clarity.

tests/setup.js

@@ -25,9 +25,9 @@ global.wait = (ms) => new Promise((resolve) => setTimeout(resolve, ms));
 if (process.env.SILENCE_CONSOLE === 'true') {
 	global.console = {
 		...console,
-		log: jest.fn(),
-		info: jest.fn(),
-		warn: jest.fn(),
-		error: jest.fn()
+		log: () => {},
+		info: () => {},
+		warn: () => {},
+		error: () => {}
 	};
 }