feat: add Cron Editor tool

Visual cron expression editor with field-by-field builder, presets
select, human-readable description, and live schedule preview showing
next occurrences. Registered in tools registry with CronIcon.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/cron/store.ts

@@ -0,0 +1,47 @@
+import { create } from 'zustand';
+import { persist } from 'zustand/middleware';
+
+export interface CronHistoryEntry {
+  id: string;
+  expression: string;
+  label?: string;
+  savedAt: number;
+}
+
+interface CronStore {
+  expression: string;
+  history: CronHistoryEntry[];
+  setExpression: (expr: string) => void;
+  addToHistory: (expr: string, label?: string) => void;
+  removeFromHistory: (id: string) => void;
+  clearHistory: () => void;
+}
+
+export const useCronStore = create<CronStore>()(
+  persist(
+    (set) => ({
+      expression: '0 9 * * 1-5',
+      history: [],
+
+      setExpression: (expression) => set({ expression }),
+
+      addToHistory: (expression, label) =>
+        set((state) => {
+          const entry: CronHistoryEntry = {
+            id: `${Date.now()}-${Math.random().toString(36).slice(2)}`,
+            expression,
+            label,
+            savedAt: Date.now(),
+          };
+          const filtered = state.history.filter((h) => h.expression !== expression);
+          return { history: [entry, ...filtered].slice(0, 30) };
+        }),
+
+      removeFromHistory: (id) =>
+        set((state) => ({ history: state.history.filter((h) => h.id !== id) })),
+
+      clearHistory: () => set({ history: [] }),
+    }),
+    { name: 'kit-cron-v1' },
+  ),
+);