feat: finish the fleets plugin, add ctx.hosts and host-commands SDK (B4)

plugins/fleets/src/frontend/helpers.ts

@@ -0,0 +1,44 @@
+import { clsx, type ClassValue } from "clsx";
+import { twMerge } from "tailwind-merge";
+
+export function cn(...inputs: ClassValue[]) {
+  return twMerge(clsx(inputs));
+}
+
+/**
+ * Extract a stable, human-readable message from an unknown thrown value.
+ * Mirrors src/ui/lib/error-message.ts - kept as a small standalone copy
+ * rather than a cross-plugin dependency for one pure function.
+ */
+export function getErrorMessage(
+  error: unknown,
+  fallback = "Unknown error",
+): string {
+  return error instanceof Error ? error.message : fallback;
+}
+
+export interface SnippetInput {
+  key: string;
+  label: string;
+}
+
+const INPUT_PATTERN =
+  /\$\{INPUT_(\d+)(?::([^}$]+))?\}|\$INPUT_(\d+)(?![a-zA-Z0-9_])/g;
+
+/**
+ * Finds the $INPUT_n placeholders in a command, for the fleet run box's
+ * input fields. Mirrors src/ui/lib/snippet-variables.ts extractSnippetInputs.
+ */
+export function extractSnippetInputs(content: string): SnippetInput[] {
+  const seen = new Map<string, SnippetInput>();
+  let match: RegExpExecArray | null;
+  INPUT_PATTERN.lastIndex = 0;
+  while ((match = INPUT_PATTERN.exec(content)) !== null) {
+    const digits = match[1] ?? match[3];
+    const key = `INPUT_${digits}`;
+    if (!seen.has(key)) {
+      seen.set(key, { key, label: match[2]?.trim() || `Input ${digits}` });
+    }
+  }
+  return Array.from(seen.values());
+}