AD-601: Define workflow DAG schema

[alph-cmky]

Summary

• Defines workflow DAG schema types for start, agent, tool, condition, human approval, and end nodes.
• Adds workflow edges, variables, permissions, status, version, and timestamp fields.
• Adds schema creation and validation helpers.
• Validates exactly one start node and at least one end node.
• Marks AD-601 complete in the development backlog.

Stack

• Stacked on AD-502: Add safe shell runtime adapter #40 (feat/ad-502-safe-shell-adapter).

Verification

• TDD red check: pnpm test packages/workflow-core/src/workflow-schema.test.ts initially failed because workflow-schema.js did not exist yet.
• pnpm test packages/workflow-core/src/workflow-schema.test.ts
• pnpm lint && pnpm format && pnpm typecheck && pnpm test && pnpm build

Closes #16