feat: Field-Level Validation

[lornakelly]

Description

Implement validation for individual field values against their own schema definitions, independent of sibling fields or broader task context.

Motivation

Users need immediate feedback when editing a single field to know if the value meets basic schema requirements (type, format, constraints) before considering relationships with other fields. This is the most granular level of validation and should execute quickly on field blur events.

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.