autoresearch iter 18: remove the phantom user:daily_audit schedule

The user plugin registered a cron dispatching to user:daily_audit, a task
pattern it never registers, and no audit logic exists anywhere in the plugin.
The daily run therefore went nowhere while a test asserted the schedule was
registered — proving the wiring existed, not that it worked. Implementing a
real daily audit is unstarted functionality, so the schedule is removed rather
than stubbed.

The combined domain test now asserts the real invariant across all applied
plugins: every schedule's task type must have a registered handler.

backend/plugins/domain/user/plugin.go

@@ -138,10 +138,7 @@ func (p *Plugin) Apply(ctx *core.Context) error {
 		return nil
 	})
 
-	// 5. Register Cron Schedules
-	ctx.Schedule().RegisterCron("0 3 * * *", "user:daily_audit", map[string]string{"type": "audit"})
-
-	// 6. Register Settings Schemas
+	// 5. Register Settings Schemas
 	ctx.Settings().Register(extpoints.SettingSchema{
 		Key:         "user.registration_enabled",
 		Default:     true,