chore: remove dev Agent Dashboard sidebar link + view constant

Follow-up to c79120ba (which only captured the file deletion): removes the
sidebar nav link and the unused DEV_DASHBOARD view constant. No dangling refs.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

components/sidebar.html

@@ -251,19 +251,6 @@
               />
               <span class="sidebar-text">βeacon</span>
             </a>
-            <a
-              href="#view=dev/agent-dashboard"
-              class="sidebar-nav-link sidebar-nav-link--sub"
-              title="Dev Tools"
-            >
-              <img
-                src="assets/svg/video-settings-gear.svg"
-                alt=""
-                aria-hidden="true"
-                class="sidebar-icon"
-              />
-              <span class="sidebar-text">Dev Tools</span>
-            </a>
             <a
               href="#"
               data-beta-link