feat: Task-Level Validation

[lornakelly]

Description

Implement validation for all fields within a task, considering the task's sub-schema and relationships between fields within that task.

Motivation

While field-level validation catches basic errors, many validation rules require understanding the context of all fields within a task. For example, conditional requirements where one field's value affects whether another field is required, or mutual exclusivity between fields.

Proposed Implementation

Refer to spike #284

Definition of Done

• Implementation: Fully implemented according to the Open Workflow spec.
• Unit Tests: Comprehensive unit tests are included and passing.
• Integration Tests: Verified within the monorepo and target environments (Web/VS Code).
• Documentation: Updated README.md, ADRs, or official docs.
• Performance: No significant regression in editor responsiveness.
• Accessibility: UI changes comply with accessibility standards.