feat: implement robust subtask validation system

- Add SubtaskValidator class for parallel validation of subtask results
- Implement validation types and interfaces
- Add validation configuration to global settings
- Create proof-of-concept integration with newTaskTool
- Add comprehensive tests for validation logic

This implements the "parallel universe" validation system proposed in issue #6970,
allowing the orchestrator to validate subtask results in a separate context before
accepting them, reducing propagated errors and improving overall reliability.

Author: roomote

packages/types/src/global-settings.ts

@@ -155,6 +155,14 @@ export const globalSettingsSchema = z.object({
 	hasOpenedModeSelector: z.boolean().optional(),
 	lastModeExportPath: z.string().optional(),
 	lastModeImportPath: z.string().optional(),
+
+	// Subtask validation configuration
+	subtaskValidationEnabled: z.boolean().optional(),
+	subtaskValidationApiConfigId: z.string().optional(),
+	subtaskValidationMaxRetries: z.number().optional(),
+	subtaskValidationAutoRevert: z.boolean().optional(),
+	subtaskValidationIncludeFullContext: z.boolean().optional(),
+	subtaskValidationCustomPrompt: z.string().optional(),
 })
 
 export type GlobalSettings = z.infer<typeof globalSettingsSchema>