feat: workspace-first UI refresh - PR-2 workspace detail page

- Add workspace detail page (/workspaces/:id) with 4 tabs:
  - Files: file tree, viewer, editor, git toolbar (commit/push/pull/fetch)
  - Git: branch selector, commit history
  - Tools: instance grid, start tool modal
  - Settings: workspace info read-only
- Add workspace API clients: workspace-files, workspace-git, workspace-instances
- Add hooks: useWorkspaceFiles, useWorkspaceGit, useWorkspaceInstances
- WorkspaceCard links to detail page via router Link
- Add comprehensive CSS for workspace detail layout
- Mobile: bottom tab bar, responsive file tree/split
- TypeScript + eslint clean

Quality gates: tsc --noEmit clean, eslint clean

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

@@ -0,0 +1,65 @@
+/** Hook for workspace instance operations. */
+
+import { useCallback, useEffect, useState } from "react";
+import {
+	listWorkspaceInstances,
+	createWorkspaceInstance,
+} from "../api/workspace-instances";
+import type { ToolInstance } from "../api/sessions";
+
+export interface UseWorkspaceInstancesResult {
+	instances: ToolInstance[];
+	loading: boolean;
+	error: string | null;
+	refresh: () => Promise<void>;
+	create: (
+		toolTypeId: string,
+		displayName?: string,
+		configProfileId?: string,
+	) => Promise<ToolInstance>;
+}
+
+export function useWorkspaceInstances(
+	workspaceId: string,
+): UseWorkspaceInstancesResult {
+	const [instances, setInstances] = useState<ToolInstance[]>([]);
+	const [loading, setLoading] = useState(true);
+	const [error, setError] = useState<string | null>(null);
+
+	const refresh = useCallback(async () => {
+		setLoading(true);
+		setError(null);
+		try {
+			const data = await listWorkspaceInstances(workspaceId);
+			setInstances(data);
+		} catch (err) {
+			setError(err instanceof Error ? err.message : "Failed to load instances");
+		} finally {
+			setLoading(false);
+		}
+	}, [workspaceId]);
+
+	const create = useCallback(
+		async (
+			toolTypeId: string,
+			displayName?: string,
+			configProfileId?: string,
+		) => {
+			const instance = await createWorkspaceInstance(
+				workspaceId,
+				toolTypeId,
+				displayName,
+				configProfileId,
+			);
+			await refresh();
+			return instance;
+		},
+		[workspaceId, refresh],
+	);
+
+	useEffect(() => {
+		refresh();
+	}, [refresh]);
+
+	return { instances, loading, error, refresh, create };
+}