feat(categories): add disable field to CategoryConfigSchema

Allow individual categories to be disabled via `disable: true` in
config. Introduce shared `mergeCategories()` utility to centralize
category merging and disabled filtering across all 7 consumption sites.

src/shared/merge-categories.ts

@@ -0,0 +1,18 @@
+import type { CategoriesConfig, CategoryConfig } from "../config/schema"
+import { DEFAULT_CATEGORIES } from "../tools/delegate-task/constants"
+
+/**
+ * Merge default and user categories, filtering out disabled ones.
+ * Single source of truth for category merging across the codebase.
+ */
+export function mergeCategories(
+  userCategories?: CategoriesConfig,
+): Record<string, CategoryConfig> {
+  const merged = userCategories
+    ? { ...DEFAULT_CATEGORIES, ...userCategories }
+    : { ...DEFAULT_CATEGORIES }
+
+  return Object.fromEntries(
+    Object.entries(merged).filter(([, config]) => !config.disable),
+  )
+}