feat(config): add is_unstable_agent option to CategoryConfig

Adds optional is_unstable_agent boolean field to CategoryConfigSchema.
When enabled (or auto-detected for gemini models), forces background
mode for monitoring stability even when run_in_background=false.

src/config/schema.ts

@@ -161,6 +161,8 @@ export const CategoryConfigSchema = z.object({
   textVerbosity: z.enum(["low", "medium", "high"]).optional(),
   tools: z.record(z.string(), z.boolean()).optional(),
   prompt_append: z.string().optional(),
+  /** Mark agent as unstable - forces background mode for monitoring. Auto-enabled for gemini models. */
+  is_unstable_agent: z.boolean().optional(),
 })
 
 export const BuiltinCategoryNameSchema = z.enum([