cleanup(dqn): Bug #3 — delete dead \if !true\ update_epsilon block (Task 2.5)

Dead code: `if !true { ... }` is a permanently-disabled guard pattern.
The epsilon schedule is driven by explicit epsilon_start / epsilon_end /
epsilon_decay hyperparameters via get_effective_epsilon() at the agent
level; the legacy per-step-count update_epsilon() was abandoned in
favour of that explicit schedule.

Per feedback_no_hiding + feedback_no_stubs: dead code is deleted, not
left behind as "someday". Per feedback_no_feature_flags: a !true toggle
is a disabled feature flag pattern.

Zero behavior change; removes foot-gun.

crates/ml/src/trainers/dqn/trainer/training_loop.rs

@@ -2992,11 +2992,13 @@ impl DQNTrainer {
             current_epsilon, epoch_duration.as_secs_f64()
         );
 
-        // Epsilon decay (skip when noisy nets)
-        if !true {
-            let mut agent = self.agent.write().await;
-            agent.update_epsilon();
-        }
+        // Task 2.5 Bug #3 — removed dead `if !true { agent.update_epsilon() }`
+        // block. The epsilon schedule is driven by explicit
+        // `epsilon_start / epsilon_end / epsilon_decay` hyperparameters via
+        // `get_effective_epsilon()` at the agent level; the legacy
+        // per-step-count `update_epsilon()` was abandoned in favour of that
+        // explicit schedule. Guarded-by-`!true` is a disabled-toggle pattern
+        // per feedback_no_feature_flags — dead code is deleted.
 
         // Noisy sigma schedule
         if let Some(ref mut scheduler) = self.noisy_sigma_scheduler {