presentation/learning-journey: stack slide 4 hallucination images and add "Fix Isn't Better Prompting" callout

Changed the slide-4 hallucination images from side-by-side to a vertically
stacked single-column grid capped at max-width 720px, and added a trigger-box
callout below them arguing that these failures (letter-counting, current date)
are fixed by structure — tools, CLAUDE.md, agents, skills — not by better
prompts. Turns the images into a forward-pointing motivation hook for the rest
of the session rather than a closed punchline.

Co-Authored-By: Claude <noreply@anthropic.com>

presentation/learning-journey/index.html

@@ -257,6 +257,14 @@ todoapp/
                 <p style="margin-top: 12px; color: #2e7d32;"><strong>Repeatable. Shareable. Same every time.</strong></p>
             </div>
         </div>
+        <div style="display: grid; grid-template-columns: 1fr; gap: 24px; margin-top: 28px; max-width: 720px; margin-left: auto; margin-right: auto;">
+            <img src="../assets/hallucinations-opus-strawperry.jpg" alt="Claude Opus hallucinating the letter count in 'strawberry'" style="width: 100%; border-radius: 12px; box-shadow: 0 4px 24px rgba(0,0,0,0.12);" />
+            <img src="../assets/hallucinations-opus-year.jpg" alt="Claude Opus hallucinating the current year" style="width: 100%; border-radius: 12px; box-shadow: 0 4px 24px rgba(0,0,0,0.12);" />
+        </div>
+        <div class="trigger-box" style="margin-top: 28px;">
+            <h4>The Fix Isn't Better Prompting</h4>
+            <p>Better prompting doesn't fix these. But giving Claude a <strong>tool to count letters</strong> and a <strong>file that says today's date</strong> does. That's what <strong>agents, skills, and CLAUDE.md</strong> are for &mdash; and that's what this session teaches.</p>
+        </div>
     </div>
 
     <!-- ============================================================ -->