AD-602: Implement Workflow Validator

[alph-cmky]

Source: docs/development/task-backlog.md

Milestone: Phase 6: Workflow Core

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AD-602: Implement Workflow Validator

Goal: Validate graph safety before execution or patch application.

Files:

• Create packages/workflow-core/src/validator.ts
• Create packages/workflow-core/src/validator.test.ts

Tasks:

• Detect cycles.
• Validate node input/output compatibility.
• Validate referenced agents and tools exist through injected resolvers.
• Validate requested permissions do not exceed workflow permissions.
• Commit with message feat: add workflow validator.

Acceptance Criteria:

• Validator returns actionable errors for schema, DAG, IO, reference, and permission failures.