Hide Teacher Model settings card (2.0 'harden the core' deferral)

The Teacher Mode feature stays out of the default UI per the 2.0
roadmap — backend escalation is already dormant when teacher_model is
unset (its default) and we want to focus on core reliability before
surfacing escalation as a feature.

Nothing removed from the backend:
- src/teacher_escalation.py still gates on get_setting('teacher_model')
- agent_loop.py's run_teacher_inline hook is a no-op without the setting
- settings backup/restore round-trips the teacher_model key unchanged
- power users can still set it via manage_settings or the JSON backup

settings.js's initTeacherModel already early-returns when the card's
DOM ids are missing, so the JS side is clean.

To re-surface the card, revert this commit.

static/index.html

@@ -1557,21 +1557,12 @@
               <div id="set-ttsSettingsMsg" style="font-size:11px;color:color-mix(in srgb, var(--fg) 45%, transparent);"></div>
             </div>
           </div>
-          <div class="admin-card">
-            <h2 style="display:flex;align-items:center;gap:6px;"><svg width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" style="vertical-align:-2px;margin-right:5px;opacity:0.6"><path d="M22 10v6M2 10l10-5 10 5-10 5z"/><path d="M6 12v5c3 3 9 3 12 0v-5"/></svg>Teacher Model <span style="font-size:0.72em;opacity:0.55;font-weight:normal;">(Experimental)</span><span style="flex:1"></span><label class="admin-switch"><input type="checkbox" id="set-teacherEnabledToggle"><span class="admin-slider"></span></label></h2>
-            <div class="admin-toggle-sub" style="margin-bottom:8px">When a self-hosted student fails an agent-mode task, escalate to a SOTA teacher that writes a SKILL.md procedure so the student can do it next time. Off by default.</div>
-            <div class="settings-col">
-              <div class="settings-row">
-                <label class="settings-label">Endpoint</label>
-                <select id="set-teacherEpSelect" class="settings-select"><option value="">—</option></select>
-              </div>
-              <div class="settings-row">
-                <label class="settings-label">Model</label>
-                <select id="set-teacherModelSelect" class="settings-select"><option value="">—</option></select>
-              </div>
-              <div id="set-teacherChatMsg" style="font-size:11px;color:color-mix(in srgb, var(--fg) 45%, transparent);"></div>
-            </div>
-          </div>
+          <!-- Teacher Model settings card hidden as part of the 2.0
+               "harden the core" pass. The escalation flow is dormant when
+               `teacher_model` is unset (its default), so the backend keeps
+               working for anyone who wired it via `manage_settings` /
+               settings backup. Re-add this card to surface the toggle
+               again once the core experience is faster. -->
         </div>
 
         <!-- ═══ SEARCH TAB ═══ -->