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.

scripts/modules/config-manager.js

@@ -338,6 +338,20 @@ function getOllamaBaseUrl(explicitRoot = null) {
 	return getGlobalConfig(explicitRoot).ollamaBaseUrl;
 }
 
+/**
+ * Gets model parameters (maxTokens, temperature) for a specific role.
+ * @param {string} role - The role ('main', 'research', 'fallback').
+ * @param {string|null} explicitRoot - Optional explicit path to the project root.
+ * @returns {{maxTokens: number, temperature: number}}
+ */
+function getParametersForRole(role, explicitRoot = null) {
+	const roleConfig = getModelConfigForRole(role, explicitRoot);
+	return {
+		maxTokens: roleConfig.maxTokens,
+		temperature: roleConfig.temperature
+	};
+}
+
 /**
  * Checks if the API key for a given provider is set in the environment.
  * Checks process.env first, then session.env if session is provided.
@@ -561,6 +575,7 @@ export {
 	getDefaultPriority,
 	getProjectName,
 	getOllamaBaseUrl,
+	getParametersForRole,
 
 	// API Key Checkers (still relevant)
 	isApiKeySet,