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/plugin/available-categories.ts

@@ -1,19 +1,14 @@
 import type { AvailableCategory } from "../agents/dynamic-agent-prompt-builder"
 import type { OhMyOpenCodeConfig } from "../config"
-
-import {
-  CATEGORY_DESCRIPTIONS,
-  DEFAULT_CATEGORIES,
-} from "../tools/delegate-task/constants"
+import { CATEGORY_DESCRIPTIONS } from "../tools/delegate-task/constants"
+import { mergeCategories } from "../shared/merge-categories"
 
 export function createAvailableCategories(
   pluginConfig: OhMyOpenCodeConfig,
 ): AvailableCategory[] {
-  const mergedCategories = pluginConfig.categories
-    ? { ...DEFAULT_CATEGORIES, ...pluginConfig.categories }
-    : DEFAULT_CATEGORIES
+  const categories = mergeCategories(pluginConfig.categories)
 
-  return Object.entries(mergedCategories).map(([name, categoryConfig]) => {
+  return Object.entries(categories).map(([name, categoryConfig]) => {
     const model =
       typeof categoryConfig.model === "string" ? categoryConfig.model : undefined