feat: work item dependency graph with blocked status and notifications

[DorianZheng]

Summary

• Adds work item dependency tracking with cycle detection and agent actions to add/remove dependencies.
• Prevents blocked work items from moving to active until dependencies reach acceptance or done.
• Adds blocked indicators, dependency management UI, and a dependency graph view for projects.
• Notifies assigned agents when a dependency resolves and their item becomes unblocked.

Validation

• pnpm exec vitest run src/shared/workflow/dependency-utils.test.ts src/electron/main/agent-actions/handlers/items.test.ts src/electron/main/agent-actions/handlers/validators.test.ts src/electron/main/agent-actions/register-actions.test.ts src/renderer/features/workflow/components/WorkflowBoard.test.tsx
• pnpm run typecheck