feat: work item dependency graph — block items until dependencies are done

[DorianZheng]

What changed

• added dependsOn support plus shared dependency utilities for blocked-state and cycle detection
• enforced dependency-aware move guards in both the renderer workflow store and main-process agent actions
• added dependency add/remove actions, assignee unblocked notifications, blocked indicators, and dependency editing in the item inspector
• added a project-level dependency graph view and tests for dependency utils, item handlers, and workflow-store guards

Why

Work items need explicit prerequisite tracking so execution cannot start before upstream work is complete.

Validation

• pnpm run typecheck
• pnpm exec vitest run src/shared/workflow/dependency-utils.test.ts src/electron/main/agent-actions/handlers/items.test.ts src/renderer/app/store/workflow-slice.test.ts src/renderer/features/workflow/components/WorkflowBoard.test.tsx src/renderer/features/workflow/components/WorkflowItemInspector.test.tsx
• pnpm exec vitest run src/renderer/features/settings/config/settings-sections.test.ts src/renderer/features/agents/runtime/agent-runtime.test.ts src/renderer/app/store/runtime-sync.test.ts src/renderer/app/AppShell.test.tsx

Notes

• opened from a clean recovery branch because the originally requested branch was mutated by an unrelated follow-up commit after push
• left unrelated unstaged worktree edits out of this branch/PR scope
• updated a stale settings-section test expectation (artifacts) encountered during verification