feat(ai): Add OpenRouter AI provider support

Integrates the OpenRouter AI provider using the Vercel AI SDK adapter (@openrouter/ai-sdk-provider). This allows users to configure and utilize models available through the OpenRouter platform.

- Added src/ai-providers/openrouter.js with standard Vercel AI SDK wrapper functions (generateText, streamText, generateObject).

- Updated ai-services-unified.js to include the OpenRouter provider in the PROVIDER_FUNCTIONS map and API key resolution logic.

- Verified config-manager.js handles OpenRouter API key checks correctly.

- Users can configure OpenRouter models via .taskmasterconfig using the task-master models command or MCP models tool. Requires OPENROUTER_API_KEY.

- Enhanced error handling in ai-services-unified.js to provide clearer messages when generateObjectService fails due to lack of underlying tool support in the selected model/provider endpoint.

.taskmasterconfig

@@ -2,7 +2,7 @@
   "models": {
     "main": {
       "provider": "openrouter",
-      "modelId": "meta-llama/llama-4-maverick:free",
+      "modelId": "google/gemini-2.5-pro-exp-03-25",
       "maxTokens": 100000,
       "temperature": 0.2
     },