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/agents/builtin-agents.ts

@@ -14,7 +14,8 @@ import { createMomusAgent, momusPromptMetadata } from "./momus"
 import { createHephaestusAgent } from "./hephaestus"
 import type { AvailableCategory } from "./dynamic-agent-prompt-builder"
 import { fetchAvailableModels, readConnectedProvidersCache } from "../shared"
-import { DEFAULT_CATEGORIES, CATEGORY_DESCRIPTIONS } from "../tools/delegate-task/constants"
+import { CATEGORY_DESCRIPTIONS } from "../tools/delegate-task/constants"
+import { mergeCategories } from "../shared/merge-categories"
 import { buildAvailableSkills } from "./builtin-agents/available-skills"
 import { collectPendingBuiltinAgents } from "./builtin-agents/general-agents"
 import { maybeCreateSisyphusConfig } from "./builtin-agents/sisyphus-agent"
@@ -78,9 +79,7 @@ export async function createBuiltinAgents(
 
   const result: Record<string, AgentConfig> = {}
 
-  const mergedCategories = categories
-    ? { ...DEFAULT_CATEGORIES, ...categories }
-    : DEFAULT_CATEGORIES
+  const mergedCategories = mergeCategories(categories)
 
   const availableCategories: AvailableCategory[] = Object.entries(mergedCategories).map(([name]) => ({
     name,