refactor(ai): Implement unified AI service layer and fix subtask update

- Unified Service: Introduced 'scripts/modules/ai-services-unified.js' to centralize AI interactions using provider modules ('src/ai-providers/') and the Vercel AI SDK.

- Provider Modules: Implemented 'anthropic.js' and 'perplexity.js' wrappers for Vercel SDK.

- 'updateSubtaskById' Fix: Refactored the AI call within 'updateSubtaskById' to use 'generateTextService' from the unified layer, resolving runtime errors related to parameter passing and streaming. This serves as the pattern for refactoring other AI calls in 'scripts/modules/task-manager/'.

- Task Status: Marked Subtask 61.19 as 'done'.

- Rules: Added new 'ai-services.mdc' rule.

This centralizes AI logic, replacing previous direct SDK calls and custom implementations. API keys are resolved via 'resolveEnvVariable' within the service layer. The refactoring of 'updateSubtaskById' establishes the standard approach for migrating other AI-dependent functions in the task manager module to use the unified service.

Relates to Task 61.

.cursor/rules/new_features.mdc

@@ -25,11 +25,17 @@ alwaysApply: false
 The standard pattern for adding a feature follows this workflow:
 
 1. **Core Logic**: Implement the business logic in the appropriate module (e.g., [`task-manager.js`](mdc:scripts/modules/task-manager.js)).
-2. **UI Components**: Add any display functions to [`ui.js`](mdc:scripts/modules/ui.js) following [`ui.mdc`](mdc:.cursor/rules/ui.mdc).
-3. **Command Integration**: Add the CLI command to [`commands.js`](mdc:scripts/modules/commands.js) following [`commands.mdc`](mdc:.cursor/rules/commands.mdc).
-4. **Testing**: Write tests for all components of the feature (following [`tests.mdc`](mdc:.cursor/rules/tests.mdc))
-5. **Configuration**: Update configuration settings or add new ones in [`config-manager.js`](mdc:scripts/modules/config-manager.js) and ensure getters/setters are appropriate. Update documentation in [`utilities.mdc`](mdc:.cursor/rules/utilities.mdc) and [`taskmaster.mdc`](mdc:.cursor/rules/taskmaster.mdc). Update the `.taskmasterconfig` structure if needed.
-6. **Documentation**: Update help text and documentation in [dev_workflow.mdc](mdc:scripts/modules/dev_workflow.mdc)
+2. **AI Integration (If Applicable)**: 
+   - Import necessary service functions (e.g., `generateTextService`, `streamTextService`) from [`ai-services-unified.js`](mdc:scripts/modules/ai-services-unified.js).
+   - Prepare parameters (`role`, `session`, `systemPrompt`, `prompt`).
+   - Call the service function.
+   - Handle the response (direct text or stream object).
+   - **Important**: Prefer `generateTextService` for calls sending large context (like stringified JSON) where incremental display is not needed. See [`ai_services.mdc`](mdc:.cursor/rules/ai_services.mdc) for detailed usage patterns and cautions.
+3. **UI Components**: Add any display functions to [`ui.js`](mdc:scripts/modules/ui.js) following [`ui.mdc`](mdc:.cursor/rules/ui.mdc).
+4. **Command Integration**: Add the CLI command to [`commands.js`](mdc:scripts/modules/commands.js) following [`commands.mdc`](mdc:.cursor/rules/commands.mdc).
+5. **Testing**: Write tests for all components of the feature (following [`tests.mdc`](mdc:.cursor/rules/tests.mdc))
+6. **Configuration**: Update configuration settings or add new ones in [`config-manager.js`](mdc:scripts/modules/config-manager.js) and ensure getters/setters are appropriate. Update documentation in [`utilities.mdc`](mdc:.cursor/rules/utilities.mdc) and [`taskmaster.mdc`](mdc:.cursor/rules/taskmaster.mdc). Update the `.taskmasterconfig` structure if needed.
+7. **Documentation**: Update help text and documentation in [`dev_workflow.mdc`](mdc:.cursor/rules/dev_workflow.mdc) and [`taskmaster.mdc`](mdc:.cursor/rules/taskmaster.mdc).
 
 ## Critical Checklist for New Features
 
@@ -211,7 +217,29 @@ export {
 ```
 
 ```javascript
-// 2. UI COMPONENTS: Add display function to ui.js
+// 2. AI Integration: Add import and use necessary service functions
+import { generateTextService } from './ai-services-unified.js';
+
+// Example usage:
+async function handleAIInteraction() {
+  const role = 'user';
+  const session = 'exampleSession';
+  const systemPrompt = 'You are a helpful assistant.';
+  const prompt = 'What is the capital of France?';
+
+  const result = await generateTextService(role, session, systemPrompt, prompt);
+  console.log(result);
+}
+
+// Export from the module
+export {
+  // ... existing exports ...
+  handleAIInteraction,
+};
+```
+
+```javascript
+// 3. UI COMPONENTS: Add display function to ui.js
 /**
  * Display archive operation results
  * @param {string} archivePath - Path to the archive file
@@ -232,7 +260,7 @@ export {
 ```
 
 ```javascript
-// 3. COMMAND INTEGRATION: Add to commands.js
+// 4. COMMAND INTEGRATION: Add to commands.js
 import { archiveTasks } from './task-manager.js';
 import { displayArchiveResults } from './ui.js';
 
@@ -452,7 +480,7 @@ npm test
 For each new feature:
 
 1. Add help text to the command definition
-2. Update [`dev_workflow.mdc`](mdc:scripts/modules/dev_workflow.mdc) with command reference
+2. Update [`dev_workflow.mdc`](mdc:.cursor/rules/dev_workflow.mdc) with command reference
 3. Consider updating [`architecture.mdc`](mdc:.cursor/rules/architecture.mdc) if the feature significantly changes module responsibilities.
 
 Follow the existing command reference format: