Add work item search and filtering

[DorianZheng]

Summary

• Add an in-memory work item search index for local Cmd+K search across titles, briefs, and work product content.
• Extend the command menu to show work item search results with title, status, assignee, project, and matching snippets while preserving keyboard navigation through cmdk.
• Add a combinable workflow board filter panel for status, assigned agent, updated date range, and reviewer feedback presence.
• Add focused unit coverage for search index matching and combined filter behavior.

How to test

• Run pnpm run typecheck.
• Run pnpm vitest run tests/unit/src/renderer/features/workflow/model/search-index.test.ts tests/unit/src/renderer/features/workflow/model/workflow-filters.test.ts tests/unit/src/renderer/app/shell/CommandMenu.test.tsx.
• Open the app, press Cmd+K, search for text from a work item title, brief, or work product, and use arrow keys/Enter to select a result.
• Open a project board and combine filters such as Status = Review, Agent = Dune Repo Lead, and Has reviewer feedback.

Notes

• pnpm run lint currently fails on existing unrelated lint debt across Electron/runtime/test files. Targeted lint for the changed files passes with pnpm eslint --ext .ts,.tsx src/renderer/app/AppShell.tsx src/renderer/app/shell/CommandMenu.tsx src/renderer/app/workspaces/WorkflowWorkspace.tsx src/renderer/features/workflow/components/FilterPanel.tsx src/renderer/features/workflow/model/search-index.ts src/renderer/features/workflow/model/workflow-filters.ts tests/unit/src/renderer/app/shell/CommandMenu.test.tsx tests/unit/src/renderer/features/workflow/model/search-index.test.ts tests/unit/src/renderer/features/workflow/model/workflow-filters.test.ts.