fix: enable folder navigation in workspace file browser

FilesTab in WorkspaceDetailPage was returning early for directories with
no action, making folders unclickable.

Changes:
- use-workspace-files.ts: add currentPath state and navigateTo() function;
  refresh() now passes currentPath to listWorkspaceFiles API
- WorkspaceDetailPage.tsx FilesTab: handleSelect now calls navigateTo()
  for directories; added navigateUp() button using '..' when not at root
- Clear selected file/editor state when changing directories

Quality gates: tsc --noEmit pass, npm run build pass, 82/82 tests pass

apps/web/src/hooks/use-workspace-files.ts

@@ -11,11 +11,13 @@ import {
 export interface UseWorkspaceFilesResult {
 	entries: FileEntry[];
 	content: string | null;
+	currentPath: string;
 	loading: boolean;
 	error: string | null;
 	refresh: () => Promise<void>;
 	loadFile: (path: string) => Promise<void>;
 	saveFile: (path: string, content: string, message?: string) => Promise<void>;
+	navigateTo: (path: string) => void;
 }
 
 export function useWorkspaceFiles(
@@ -23,6 +25,7 @@ export function useWorkspaceFiles(
 ): UseWorkspaceFilesResult {
 	const [entries, setEntries] = useState<FileEntry[]>([]);
 	const [content, setContent] = useState<string | null>(null);
+	const [currentPath, setCurrentPath] = useState("");
 	const [loading, setLoading] = useState(true);
 	const [error, setError] = useState<string | null>(null);
 
@@ -30,14 +33,19 @@ export function useWorkspaceFiles(
 		setLoading(true);
 		setError(null);
 		try {
-			const data = await listWorkspaceFiles(workspaceId);
+			const data = await listWorkspaceFiles(workspaceId, currentPath);
 			setEntries(data);
 		} catch (err) {
 			setError(err instanceof Error ? err.message : "Failed to load files");
 		} finally {
 			setLoading(false);
 		}
-	}, [workspaceId]);
+	}, [workspaceId, currentPath]);
+
+	const navigateTo = useCallback((path: string) => {
+		setCurrentPath(path);
+		setContent(null);
+	}, []);
 
 	const loadFile = useCallback(
 		async (path: string) => {
@@ -64,5 +72,5 @@ export function useWorkspaceFiles(
 		refresh();
 	}, [refresh]);
 
-	return { entries, content, loading, error, refresh, loadFile, saveFile };
+	return { entries, content, currentPath, loading, error, refresh, loadFile, saveFile, navigateTo };
 }