Fix categories not being deep merged in mergeConfigs

When merging user and project configs, categories were simply
spread instead of deep merged. This caused user-level category
model settings to be completely overwritten by project-level
configs, even when the project config only specified partial
overrides like temperature.

Add deepMerge for categories field and comprehensive tests.

src/plugin-config.ts

@@ -55,6 +55,7 @@ export function mergeConfigs(
     ...base,
     ...override,
     agents: deepMerge(base.agents, override.agents),
+    categories: deepMerge(base.categories, override.categories),
     disabled_agents: [
       ...new Set([
         ...(base.disabled_agents ?? []),