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/fixtures/.taskmasterconfig

@@ -0,0 +1,16 @@
+{
+  "models": {
+    "main": {
+      "provider": "openai",
+      "modelId": "gpt-4o"
+    },
+    "research": {
+      "provider": "perplexity",
+      "modelId": "sonar-pro"
+    },
+    "fallback": {
+      "provider": "anthropic",
+      "modelId": "claude-3-haiku-20240307"
+    }
+  }
+}