feat(KB-029): add file browser and editor with CodeMirror 6 integration

- Add server-side file service with list, read, and write operations

- Add file API routes with comprehensive tests

- Create useFileBrowser and useFileEditor hooks for state management

- Add FileBrowser, FileEditor, and FileBrowserModal components

- Integrate Files tab into TaskDetailModal for task worktree browsing

- Add CodeMirror 6 dependencies for syntax highlighting

- Include file browser styles with light/dark theme support

packages/dashboard/app/styles.css

@@ -3497,8 +3497,6 @@ body {
   font-style: italic;
 }
 
-/* Model selector filter styles */
-.model-selector-filter {
   display: flex;
   align-items: center;
   gap: 8px;
@@ -4288,7 +4286,6 @@ html .column.drag-over * {
 }
 
 .file-node-time {
-  font-size: 11px;
   color: var(--text-dim);
   font-family: "SF Mono", Monaco, Consolas, monospace;
   flex-shrink: 0;