feat: Work item priority and SLA deadlines with escalation alerts

[DorianZheng]

Summary

• Add work item priority levels (critical, high, medium, low) and SLA deadline metadata through workflow persistence, normalization, IPC/action handlers, and renderer state.
• Add UI priority badges, SLA countdown/met/breached states, and lane ordering by priority then most recent update.
• Add the background SLA monitor with warning/breach notifications and workflow audit/activity events for priority and SLA changes.
• Add migration/schema support for priority and SLA columns plus focused unit coverage for actions, monitor behavior, countdown state, card rendering, and lane sorting.

Validation

• pnpm tsc --noEmit
• pnpm vitest run tests/unit/src/electron/main/sla/sla-monitor.test.ts tests/unit/src/electron/main/agent-actions/handlers/items-priority-sla.test.ts tests/unit/src/renderer/features/workflow/hooks/use-sla-countdown.test.ts tests/unit/src/renderer/features/workflow/components/ItemCard.test.tsx tests/unit/src/renderer/features/workflow/components/Lane.test.tsx tests/unit/src/renderer/features/workflow/components/WorkflowBoard.test.tsx tests/unit/src/electron/main/runtime/desktop-runtime-controller.test.ts
• pnpm vitest run has the known 8 failures outside this feature area.

Note

• origin/feature/priority-sla already existed with a different history, so this clean main-based implementation was pushed as feature/priority-sla-main instead of force-pushing over the existing branch.