AD-601: Define Workflow DAG Schema

[alph-cmky]

Source: docs/development/task-backlog.md

Milestone: Phase 6: Workflow Core

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AD-601: Define Workflow DAG Schema

Goal: Implement the workflow model described in the product design.

Files:

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

Tasks:

• Define node types: start, agent, tool, condition, humanApproval, and end.
• Define edges, variables, permissions, status, version, and timestamps.
• Validate exactly one start node and at least one end node.
• Commit with message feat: define workflow dag schema.

Acceptance Criteria:

• Invalid node types, missing start node, and missing end node fail validation.