feat(babysitting): make unstable-agent-babysitter always-on by default

Remove the 'enabled' flag from babysitting config - the hook now runs
automatically when not disabled via disabled_hooks. This simplifies
configuration and makes the unstable model monitoring a default behavior.

BREAKING CHANGE: babysitting.enabled config option is removed. Use
disabled_hooks: ['unstable-agent-babysitter'] to disable the hook instead.

src/config/schema.ts

@@ -313,7 +313,6 @@ export const NotificationConfigSchema = z.object({
 })
 
 export const BabysittingConfigSchema = z.object({
-  enabled: z.boolean().default(false),
   timeout_ms: z.number().default(120000),
 })